diff --git a/.travis.yml b/.travis.yml
index d2dacad..00caa32 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,5 @@
 language: php
 php:
-  - 5.6
-  - 7.0
   - 7.1
   - 7.2
   - nightly
diff --git a/composer.json b/composer.json
index 2816a16..98b0bca 100755
--- a/composer.json
+++ b/composer.json
@@ -12,8 +12,11 @@
   ],
   "minimum-stability": "dev",
   "require": {
-    "php": ">=5.6",
-    "ext-mbstring": "*"
+    "php": "^7.1.3",
+    "ext-mbstring": "*",
+    "ext-json": "*",
+    "ext-simplexml": "*",
+    "ext-dom": "*"
   },
   "require-dev": {
     "phpunit/phpunit": "^5.7 || ^6.5 || ^7.0",
diff --git a/src/Contracts/ConverterHandlerInterface.php b/src/Contracts/ConverterHandlerInterface.php
index 23f0053..2aad379 100755
--- a/src/Contracts/ConverterHandlerInterface.php
+++ b/src/Contracts/ConverterHandlerInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Contracts;
 
 interface ConverterHandlerInterface
@@ -10,7 +12,7 @@ interface ConverterHandlerInterface
      *
      * @return string
      */
-    public function getExtension();
+    public function getExtension(): string;
 
     /**
      * Does the actual conversion. This is where the actual conversion
@@ -18,9 +20,9 @@ public function getExtension();
      *
      * @param array $data
      *
-     * @return $this
+     * @return self
      */
-    public function convert($data);
+    public function convert(array $data): self;
 
     /**
      * Writes the converted data to the path specified.
@@ -29,5 +31,5 @@ public function convert($data);
      *
      * @return bool
      */
-    public function write($filename);
+    public function write(string $filename): bool;
 }
diff --git a/src/Contracts/ValidationRuleInterface.php b/src/Contracts/ValidationRuleInterface.php
index 2bc1285..c074106 100644
--- a/src/Contracts/ValidationRuleInterface.php
+++ b/src/Contracts/ValidationRuleInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Contracts;
 
 interface ValidationRuleInterface
@@ -11,7 +13,7 @@ interface ValidationRuleInterface
      *
      * @return int
      */
-    public function parameterCount();
+    public function parameterCount(): int;
 
     /**
      * Determines if the validation rule passes. This is where we do the
@@ -22,7 +24,7 @@ public function parameterCount();
      *
      * @return bool
      */
-    public function passes($value, $parameters);
+    public function passes($value, array $parameters): bool;
 
     /**
      * Get the validation error message. Specify the message that should
@@ -31,7 +33,7 @@ public function passes($value, $parameters);
      *
      * @return string
      */
-    public function message();
+    public function message(): string;
 
     /**
      * Replace error messages parameter with right values. If you want
@@ -49,5 +51,5 @@ public function message();
      *
      * @return string
      */
-    public function parameterReplacer($message, $parameters);
+    public function parameterReplacer(string $message, array $parameters): string;
 }
diff --git a/src/Converter/JsonConverter.php b/src/Converter/JsonConverter.php
index 185617b..cf1a03e 100755
--- a/src/Converter/JsonConverter.php
+++ b/src/Converter/JsonConverter.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Converter;
 
 use Oshomo\CsvUtils\Contracts\ConverterHandlerInterface;
@@ -18,7 +20,7 @@ class JsonConverter implements ConverterHandlerInterface
     /**
      * @return string
      */
-    public function getExtension()
+    public function getExtension(): string
     {
         return self::FILE_EXTENSION;
     }
@@ -26,9 +28,9 @@ public function getExtension()
     /**
      * @param array $data
      *
-     * @return $this|mixed
+     * @return ConverterHandlerInterface
      */
-    public function convert($data)
+    public function convert(array $data): ConverterHandlerInterface
     {
         $this->data = json_encode(
             $data,
@@ -46,7 +48,7 @@ public function convert($data)
      *
      * @return bool
      */
-    public function write($filename)
+    public function write(string $filename): bool
     {
         return (file_put_contents($filename, $this->data)) ? true : false;
     }
diff --git a/src/Converter/XmlConverter.php b/src/Converter/XmlConverter.php
index 6cb5b00..3ed8e54 100755
--- a/src/Converter/XmlConverter.php
+++ b/src/Converter/XmlConverter.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Converter;
 
 use DOMDocument;
@@ -29,28 +31,28 @@ class XmlConverter implements ConverterHandlerInterface
      *
      * @param string $recordElement
      */
-    public function __construct($recordElement = self::DEFAULT_RECORD_ELEMENT)
+    public function __construct(string $recordElement = self::DEFAULT_RECORD_ELEMENT)
     {
         if (!empty($recordElement)) {
             $this->recordElement = $recordElement;
         }
 
-        $this->data = new SimpleXMLElement('<?xml version="1.0"?><data></data>');
+        $this->data = new SimpleXMLElement('<?xml version="1.0"?><data value=""></data>');
     }
 
     /**
      * @return string
      */
-    public function getExtension()
+    public function getExtension(): string
     {
         return self::FILE_EXTENSION;
     }
 
     /**
-     * @param $data
-     * @param $xmlData
+     * @param array            $data
+     * @param SimpleXMLElement $xmlData
      */
-    protected function toXml($data, $xmlData)
+    protected function toXml(array $data, SimpleXMLElement $xmlData): void
     {
         foreach ($data as $key => $value) {
             if (is_numeric($key)) {
@@ -66,11 +68,11 @@ protected function toXml($data, $xmlData)
     }
 
     /**
-     * @param $data
+     * @param array $data
      *
-     * @return $this|mixed
+     * @return ConverterHandlerInterface
      */
-    public function convert($data)
+    public function convert(array $data): ConverterHandlerInterface
     {
         $this->toXml($data, $this->data);
 
@@ -78,11 +80,11 @@ public function convert($data)
     }
 
     /**
-     * @param $filename
+     * @param string $filename
      *
      * @return bool
      */
-    public function write($filename)
+    public function write(string $filename): bool
     {
         $dom = new DOMDocument('1.0');
 
diff --git a/src/Helpers/FormatsMessages.php b/src/Helpers/FormatsMessages.php
index 737e6b7..439911c 100755
--- a/src/Helpers/FormatsMessages.php
+++ b/src/Helpers/FormatsMessages.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Helpers;
 
 use Oshomo\CsvUtils\Contracts\ValidationRuleInterface;
@@ -15,7 +17,7 @@ trait FormatsMessages
      *
      * @return string
      */
-    protected function getMessage($attribute, $rule, $actualRule)
+    protected function getMessage(string $attribute, ValidationRuleInterface $rule, string $actualRule): string
     {
         $inlineMessage = $this->getInlineMessage($attribute, $actualRule);
 
@@ -34,7 +36,7 @@ protected function getMessage($attribute, $rule, $actualRule)
      *
      * @return string|null
      */
-    protected function getInlineMessage($attribute, $rule)
+    protected function getInlineMessage(string $attribute, string $rule): ?string
     {
         return $this->getFromLocalArray($attribute, $this->ruleToLower($rule));
     }
@@ -47,7 +49,7 @@ protected function getInlineMessage($attribute, $rule)
      *
      * @return string|null
      */
-    protected function getFromLocalArray($attribute, $lowerRule)
+    protected function getFromLocalArray(string $attribute, string $lowerRule): ?string
     {
         $source = $this->customMessages;
 
@@ -60,14 +62,16 @@ protected function getFromLocalArray($attribute, $lowerRule)
                 }
             }
         }
+
+        return null;
     }
 
     /**
-     * @param $rule
+     * @param string $rule
      *
      * @return string|string[]|null
      */
-    protected function ruleToLower($rule)
+    protected function ruleToLower(string $rule): ?string
     {
         $lowerRule = preg_replace('/[A-Z]/', '_$0', $rule);
 
@@ -86,11 +90,11 @@ protected function ruleToLower($rule)
      * @param mixed                   $value
      * @param ValidationRuleInterface $rule
      * @param array                   $parameters
-     * @param $lineNumber
+     * @param int                     $lineNumber
      *
      * @return string
      */
-    protected function makeReplacements($message, $attribute, $value, $rule, $parameters, $lineNumber)
+    protected function makeReplacements(string $message, string $attribute, $value, ValidationRuleInterface $rule, array $parameters, int $lineNumber): string
     {
         $message = $this->replaceAttributePlaceholder($message, $attribute);
 
@@ -111,7 +115,7 @@ protected function makeReplacements($message, $attribute, $value, $rule, $parame
      *
      * @return string
      */
-    protected function replaceAttributePlaceholder($message, $attribute)
+    protected function replaceAttributePlaceholder(string $message, string $attribute): string
     {
         return str_replace([':attribute'], [$attribute], $message);
     }
@@ -124,7 +128,7 @@ protected function replaceAttributePlaceholder($message, $attribute)
      *
      * @return string
      */
-    protected function replaceValuePlaceholder($message, $value)
+    protected function replaceValuePlaceholder(string $message, string $value): string
     {
         return str_replace([':value'], [$value], $message);
     }
@@ -132,13 +136,13 @@ protected function replaceValuePlaceholder($message, $value)
     /**
      * Replace the :line placeholder in the given message.
      *
-     * @param $message
-     * @param $lineNUmber
+     * @param string $message
+     * @param int    $lineNumber
      *
      * @return mixed
      */
-    protected function replaceErrorLinePlaceholder($message, $lineNUmber)
+    protected function replaceErrorLinePlaceholder(string $message, int $lineNumber)
     {
-        return str_replace([':line'], [$lineNUmber], $message);
+        return str_replace([':line'], [$lineNumber], $message);
     }
 }
diff --git a/src/Rules/AsciiOnly.php b/src/Rules/AsciiOnly.php
index 045706f..cea5171 100644
--- a/src/Rules/AsciiOnly.php
+++ b/src/Rules/AsciiOnly.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Rules;
 
 use Oshomo\CsvUtils\Contracts\ValidationRuleInterface;
@@ -11,7 +13,7 @@ class AsciiOnly implements ValidationRuleInterface
      *
      * @return int
      */
-    public function parameterCount()
+    public function parameterCount(): int
     {
         return 0;
     }
@@ -20,11 +22,11 @@ public function parameterCount()
      * Determine if the validation rule passes.
      *
      * @param mixed $value
-     * @param $parameters
+     * @param array $parameters
      *
      * @return bool
      */
-    public function passes($value, $parameters)
+    public function passes($value, array $parameters): bool
     {
         return (mb_detect_encoding($value, 'ASCII', true)) ? true : false;
     }
@@ -34,7 +36,7 @@ public function passes($value, $parameters)
      *
      * @return string
      */
-    public function message()
+    public function message(): string
     {
         return 'The :attribute value :value contains a non-ascii character on line :line.';
     }
@@ -47,7 +49,7 @@ public function message()
      *
      * @return string
      */
-    public function parameterReplacer($message, $parameters)
+    public function parameterReplacer(string $message, array $parameters): string
     {
         return $message;
     }
diff --git a/src/Rules/Between.php b/src/Rules/Between.php
index 6255cff..42bfe59 100644
--- a/src/Rules/Between.php
+++ b/src/Rules/Between.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Rules;
 
 use Oshomo\CsvUtils\Contracts\ValidationRuleInterface;
@@ -13,7 +15,7 @@ class Between implements ValidationRuleInterface
      *
      * @return int
      */
-    public function parameterCount()
+    public function parameterCount(): int
     {
         return self::PARAMETER_COUNT;
     }
@@ -22,11 +24,11 @@ public function parameterCount()
      * Determine if the validation rule passes.
      *
      * @param mixed $value
-     * @param $parameters
+     * @param array $parameters
      *
      * @return bool
      */
-    public function passes($value, $parameters)
+    public function passes($value, array $parameters): bool
     {
         $size = (int) $value;
 
@@ -38,7 +40,7 @@ public function passes($value, $parameters)
      *
      * @return string
      */
-    public function message()
+    public function message(): string
     {
         return 'The :attribute value :value is not between :min - :max on line :line.';
     }
@@ -51,7 +53,7 @@ public function message()
      *
      * @return string
      */
-    public function parameterReplacer($message, $parameters)
+    public function parameterReplacer(string $message, array $parameters): string
     {
         return str_replace([':min', ':max'], [$parameters[0], $parameters[1]], $message);
     }
diff --git a/src/Rules/ClosureValidationRule.php b/src/Rules/ClosureValidationRule.php
index 8dddb5e..c531448 100755
--- a/src/Rules/ClosureValidationRule.php
+++ b/src/Rules/ClosureValidationRule.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Rules;
 
 use Oshomo\CsvUtils\Contracts\ValidationRuleInterface;
@@ -32,7 +34,7 @@ class ClosureValidationRule implements ValidationRuleInterface
      *
      * @param \Closure $callback
      */
-    public function __construct($callback)
+    public function __construct(\Closure $callback)
     {
         $this->callback = $callback;
     }
@@ -42,7 +44,7 @@ public function __construct($callback)
      *
      * @return int
      */
-    public function parameterCount()
+    public function parameterCount(): int
     {
         return 0;
     }
@@ -51,11 +53,11 @@ public function parameterCount()
      * Determine if the validation rule passes.
      *
      * @param mixed $value
-     * @param $parameters
+     * @param array $parameters
      *
      * @return bool
      */
-    public function passes($value, $parameters)
+    public function passes($value, array $parameters): bool
     {
         $this->failed = false;
 
@@ -73,7 +75,7 @@ public function passes($value, $parameters)
      *
      * @return string
      */
-    public function message()
+    public function message(): string
     {
         return $this->message;
     }
@@ -86,7 +88,7 @@ public function message()
      *
      * @return string
      */
-    public function parameterReplacer($message, $parameters)
+    public function parameterReplacer(string $message, array $parameters): string
     {
         return $message;
     }
diff --git a/src/Rules/Url.php b/src/Rules/Url.php
index 3fdf0a1..d07e45d 100644
--- a/src/Rules/Url.php
+++ b/src/Rules/Url.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Rules;
 
 use Oshomo\CsvUtils\Contracts\ValidationRuleInterface;
@@ -11,7 +13,7 @@ class Url implements ValidationRuleInterface
      *
      * @return int
      */
-    public function parameterCount()
+    public function parameterCount(): int
     {
         return 0;
     }
@@ -20,11 +22,11 @@ public function parameterCount()
      * Determine if the validation rule passes.
      *
      * @param mixed $value
-     * @param $parameters
+     * @param array $parameters
      *
      * @return bool
      */
-    public function passes($value, $parameters)
+    public function passes($value, array $parameters): bool
     {
         /*
         * This pattern is derived from Symfony\Component\Validator\Constraints\UrlValidator (2.7.4).
@@ -55,7 +57,7 @@ public function passes($value, $parameters)
      *
      * @return string
      */
-    public function message()
+    public function message(): string
     {
         return 'The :attribute value :value is not a valid url on line :line.';
     }
@@ -68,7 +70,7 @@ public function message()
      *
      * @return string
      */
-    public function parameterReplacer($message, $parameters)
+    public function parameterReplacer(string $message, array $parameters): string
     {
         return $message;
     }
diff --git a/src/Validator/ValidationRuleParser.php b/src/Validator/ValidationRuleParser.php
index 555d6c2..65f6831 100755
--- a/src/Validator/ValidationRuleParser.php
+++ b/src/Validator/ValidationRuleParser.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Validator;
 
 use Closure;
@@ -15,7 +17,7 @@ class ValidationRuleParser
      *
      * @return array
      */
-    public static function parse($rule)
+    public static function parse($rule): array
     {
         if ($rule instanceof Closure) {
             return [new ClosureValidationRule($rule), []];
@@ -35,7 +37,7 @@ public static function parse($rule)
      *
      * @return array
      */
-    protected static function parseStringRule($rule)
+    protected static function parseStringRule(string $rule): array
     {
         $parameters = [];
 
@@ -58,7 +60,7 @@ protected static function parseStringRule($rule)
      *
      * @return array
      */
-    protected static function parseParameters($parameter)
+    protected static function parseParameters(string $parameter): array
     {
         return str_getcsv($parameter);
     }
@@ -70,7 +72,7 @@ protected static function parseParameters($parameter)
      *
      * @return string
      */
-    protected static function normalizeRule($rule)
+    protected static function normalizeRule(string $rule): string
     {
         $rule = ucwords(str_replace(['-', '_'], ' ', $rule));
 
diff --git a/src/Validator/Validator.php b/src/Validator/Validator.php
index d4f413e..9997ea8 100755
--- a/src/Validator/Validator.php
+++ b/src/Validator/Validator.php
@@ -1,9 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Oshomo\CsvUtils\Validator;
 
 use Oshomo\CsvUtils\Contracts\ConverterHandlerInterface;
-use Oshomo\CsvUtils\Contracts\ConverterHandlerInterface as Converter;
 use Oshomo\CsvUtils\Contracts\ValidationRuleInterface;
 use Oshomo\CsvUtils\Contracts\ValidationRuleInterface as ValidationRule;
 use Oshomo\CsvUtils\Helpers\FormatsMessages;
@@ -117,7 +118,7 @@ class Validator
      * @param array  $rules
      * @param array  $messages
      */
-    public function __construct($filePath, $delimiter, array $rules, array $messages = [])
+    public function __construct(string $filePath, string $delimiter = ',', array $rules, array $messages = [])
     {
         $this->filePath = $filePath;
         $this->delimiter = $delimiter;
@@ -130,8 +131,10 @@ public function __construct($filePath, $delimiter, array $rules, array $messages
 
     /**
      * Run the validator's rules against the supplied data.
+     *
+     * @return array
      */
-    public function validate()
+    public function validate(): array
     {
         if ($this->fails()) {
             return $this->errors();
@@ -145,8 +148,10 @@ public function validate()
 
     /**
      * Return validation errors.
+     *
+     * @return array
      */
-    public function errors()
+    public function errors(): array
     {
         if (empty($this->message) && empty($this->invalidRows)) {
             $message = self::NO_ERROR_MESSAGE;
@@ -167,7 +172,7 @@ public function errors()
      *
      * @return bool
      */
-    public function fails()
+    public function fails(): bool
     {
         return !$this->passes();
     }
@@ -177,7 +182,7 @@ public function fails()
      *
      * @return bool
      */
-    protected function passes()
+    protected function passes(): bool
     {
         if ($this->doesFileExistAndReadable($this->filePath)) {
             if (false !== ($handle = fopen($this->filePath, 'r'))) {
@@ -212,7 +217,7 @@ protected function passes()
      *
      * @return bool
      */
-    public function write(Converter $format)
+    public function write(ConverterHandlerInterface $format): bool
     {
         return $format
             ->convert($this->data)
@@ -222,7 +227,7 @@ public function write(Converter $format)
     /**
      * Set CSV filename.
      */
-    protected function setFileName()
+    protected function setFileName(): void
     {
         $this->fileName = basename($this->filePath, self::FILE_EXTENSION);
     }
@@ -230,7 +235,7 @@ protected function setFileName()
     /**
      * Set CSV file directory.
      */
-    protected function setFileDirectory()
+    protected function setFileDirectory(): void
     {
         $this->directory = dirname($this->filePath) . DIRECTORY_SEPARATOR;
     }
@@ -238,11 +243,11 @@ protected function setFileDirectory()
     /**
      * Get the full path and name of the file to be written.
      *
-     * @param $extension
+     * @param string $extension
      *
      * @return string
      */
-    protected function getWriteFileName($extension)
+    protected function getWriteFileName(string $extension): string
     {
         return $this->directory . $this->fileName . '.' . $extension;
     }
@@ -250,9 +255,9 @@ protected function getWriteFileName($extension)
     /**
      * Validate a given row with the supplied  rules.
      *
-     * @param $row
+     * @param array $row
      */
-    protected function validateRow($row)
+    protected function validateRow(array $row): void
     {
         $this->currentRowMessages = [];
         $this->currentRow = $row;
@@ -274,16 +279,14 @@ protected function validateRow($row)
     /**
      * Validate a given attribute against a rule.
      *
-     * @param string $attribute
-     * @param string $rule
-     *
-     * @return void|null
+     * @param string        $attribute
+     * @param string|object $rule
      */
-    protected function validateAttribute($attribute, $rule)
+    protected function validateAttribute(string $attribute, $rule): void
     {
         list($rule, $parameters) = ValidationRuleParser::parse($rule);
 
-        if ('' == $rule) {
+        if ('' === $rule) {
             return;
         }
 
@@ -306,15 +309,17 @@ protected function validateAttribute($attribute, $rule)
                     $parameters
                 );
             }
+
+            return;
         }
     }
 
     /**
-     * @param $filePath
+     * @param string $filePath
      *
      * @return bool
      */
-    protected function doesFileExistAndReadable($filePath)
+    protected function doesFileExistAndReadable(string $filePath): bool
     {
         return file_exists($filePath) && is_readable($filePath);
     }
@@ -322,7 +327,7 @@ protected function doesFileExistAndReadable($filePath)
     /**
      * @param array $headers
      */
-    protected function setHeaders($headers)
+    protected function setHeaders(array $headers): void
     {
         $this->headers = $headers;
     }
@@ -331,24 +336,23 @@ protected function setHeaders($headers)
      * Determine if the attribute is validate-able.
      *
      * @param object|string $rule
-     * @param string        $parameters
+     * @param array         $parameters
      *
      * @return bool
      */
-    protected function isValidateAble($rule, $parameters)
+    protected function isValidateAble($rule, array $parameters): bool
     {
-        return $this->ruleExists($rule) &&
-            $this->passesParameterCheck($rule, $parameters);
+        return $this->ruleExists($rule) && $this->passesParameterCheck($rule, $parameters);
     }
 
     /**
      * Get the class of a rule.
      *
-     * @param $rule
+     * @param string $rule
      *
      * @return string
      */
-    protected function getRuleClassName($rule)
+    protected function getRuleClassName(string $rule): string
     {
         return 'Oshomo\\CsvUtils\\Rules\\' . $rule;
     }
@@ -356,11 +360,11 @@ protected function getRuleClassName($rule)
     /**
      * Get the class of a rule.
      *
-     * @param $rule
+     * @param string $rule
      *
      * @return ValidationRuleInterface
      */
-    protected function getRuleClass($rule)
+    protected function getRuleClass(string $rule): ValidationRuleInterface
     {
         $ruleClassName = $this->getRuleClassName($rule);
 
@@ -374,21 +378,20 @@ protected function getRuleClass($rule)
      *
      * @return bool
      */
-    protected function ruleExists($rule)
+    protected function ruleExists($rule): bool
     {
-        return $rule instanceof ValidationRule ||
-            class_exists($this->getRuleClassName($rule));
+        return $rule instanceof ValidationRule || class_exists($this->getRuleClassName($rule));
     }
 
     /**
      * Determine if a given rule expect parameters and that the parameters where sent.
      *
      * @param object|string $rule
-     * @param $parameters
+     * @param array         $parameters
      *
      * @return bool
      */
-    protected function passesParameterCheck($rule, $parameters)
+    protected function passesParameterCheck($rule, array $parameters): bool
     {
         if (!$rule instanceof ValidationRule) {
             $rule = $this->getRuleClass($rule);
@@ -403,12 +406,12 @@ protected function passesParameterCheck($rule, $parameters)
     /**
      * Validate an attribute using a custom rule object.
      *
-     * @param string $attribute
-     * @param mixed  $value
-     * @param $parameters
+     * @param string                  $attribute
+     * @param mixed                   $value
+     * @param array                   $parameters
      * @param ValidationRuleInterface $rule
      */
-    protected function validateUsingCustomRule($attribute, $value, $parameters, $rule)
+    protected function validateUsingCustomRule(string $attribute, $value, array $parameters, ValidationRuleInterface $rule): void
     {
         if (!$rule->passes($value, $parameters)) {
             $this->addFailure($rule->message(), $attribute, $value, $rule, $parameters);
@@ -418,13 +421,13 @@ protected function validateUsingCustomRule($attribute, $value, $parameters, $rul
     /**
      * Add a failed rule and error message to the collection.
      *
-     * @param $message
+     * @param string                  $message
      * @param string                  $attribute
      * @param mixed                   $value
      * @param ValidationRuleInterface $rule
      * @param array                   $parameters
      */
-    protected function addFailure($message, $attribute, $value, $rule, $parameters = [])
+    protected function addFailure(string $message, string $attribute, $value, ValidationRuleInterface $rule, array $parameters = []): void
     {
         $this->currentRowMessages[] = $this->makeReplacements(
             $message,
@@ -443,7 +446,7 @@ protected function addFailure($message, $attribute, $value, $rule, $parameters =
      *
      * @return mixed
      */
-    protected function getValue($attribute)
+    protected function getValue(string $attribute)
     {
         return $this->currentRow[$attribute];
     }
diff --git a/tests/data/valid_test_expected.xml b/tests/data/valid_test_expected.xml
index ca98844..dde0701 100644
--- a/tests/data/valid_test_expected.xml
+++ b/tests/data/valid_test_expected.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0"?>
-<data>
+<data value="">
   <item>
     <name>Well Health Hotels</name>
     <address>Inga N. P.O. Box 567</address>
diff --git a/tests/data/valid_test_param_expected.xml b/tests/data/valid_test_param_expected.xml
index a8d64e4..86a7d0e 100644
--- a/tests/data/valid_test_param_expected.xml
+++ b/tests/data/valid_test_param_expected.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0"?>
-<data>
+<data value="">
   <sample>
     <name>Well Health Hotels</name>
     <address>Inga N. P.O. Box 567</address>
diff --git a/tests/src/Rules/ClosureValidationTest.php b/tests/src/Rules/ClosureValidationTest.php
new file mode 100644
index 0000000..e399133
--- /dev/null
+++ b/tests/src/Rules/ClosureValidationTest.php
@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Oshomo\CsvUtils\Tests\src\Rules;
+
+use Oshomo\CsvUtils\Rules\ClosureValidationRule;
+use PHPUnit\Framework\TestCase;
+
+class ClosureValidationTest extends TestCase
+{
+    public function testParemetersCount()
+    {
+        $closureValidationRule = new ClosureValidationRule(function () {
+            return;
+        });
+
+        $this->assertEquals(0, $closureValidationRule->parameterCount());
+    }
+}
diff --git a/tests/src/UppercaseRule.php b/tests/src/UppercaseRule.php
index 1ba1fd7..461c5b4 100644
--- a/tests/src/UppercaseRule.php
+++ b/tests/src/UppercaseRule.php
@@ -9,7 +9,7 @@ class UppercaseRule implements ValidationRuleInterface
     /**
      * @return int
      */
-    public function parameterCount()
+    public function parameterCount(): int
     {
         return 0;
     }
@@ -20,7 +20,7 @@ public function parameterCount()
      *
      * @return bool
      */
-    public function passes($value, $parameters)
+    public function passes($value, array $parameters): bool
     {
         return strtoupper($value) === $value;
     }
@@ -28,7 +28,7 @@ public function passes($value, $parameters)
     /**
      * @return string
      */
-    public function message()
+    public function message(): string
     {
         return 'The :attribute value :value must be uppercase on line :line.';
     }
@@ -39,7 +39,7 @@ public function message()
      *
      * @return string
      */
-    public function parameterReplacer($message, $parameters)
+    public function parameterReplacer(string $message, array $parameters): string
     {
         return $message;
     }