diff --git a/src/Client/MessageClient.php b/src/Client/MessageClient.php
index 4bb6d30..fe06ceb 100644
--- a/src/Client/MessageClient.php
+++ b/src/Client/MessageClient.php
@@ -40,7 +40,7 @@ public function buildApiUrl(): string
     /**
      * Builds array for CURLOPT_POSTFIELDS curl argument.
      *
-     * @return array<string, (null|\CURLFile|int|string)>
+     * @return array<string, (null|\CURLFile|string)>
      */
     public function buildCurlPostFields(Notification $notification): array
     {
@@ -68,11 +68,11 @@ public function buildCurlPostFields(Notification $notification): array
         }
 
         if (null !== $notification->getMessage()->getPriority()) {
-            $curlPostFields['priority'] = $notification->getMessage()->getPriority()->getPriority();
+            $curlPostFields['priority'] = (string) $notification->getMessage()->getPriority()->getPriority();
 
             if (Priority::EMERGENCY === $notification->getMessage()->getPriority()->getPriority()) {
-                $curlPostFields['retry'] = $notification->getMessage()->getPriority()->getRetry();
-                $curlPostFields['expire'] = $notification->getMessage()->getPriority()->getExpire();
+                $curlPostFields['retry'] = (string) $notification->getMessage()->getPriority()->getRetry();
+                $curlPostFields['expire'] = (string) $notification->getMessage()->getPriority()->getExpire();
 
                 if (null !== $notification->getMessage()->getPriority()->getCallback()) {
                     $curlPostFields['callback'] = $notification->getMessage()->getPriority()->getCallback();
@@ -81,11 +81,11 @@ public function buildCurlPostFields(Notification $notification): array
         }
 
         if (true === $notification->getMessage()->getIsHtml()) {
-            $curlPostFields['html'] = 1;
+            $curlPostFields['html'] = '1';
         }
 
         if (null !== $notification->getMessage()->getTtl()) {
-            $curlPostFields['ttl'] = $notification->getMessage()->getTtl();
+            $curlPostFields['ttl'] = (string) $notification->getMessage()->getTtl();
         }
 
         if (null !== $notification->getSound()) {
diff --git a/tests/Client/MessageClientTest.php b/tests/Client/MessageClientTest.php
index 75ec075..35b3579 100644
--- a/tests/Client/MessageClientTest.php
+++ b/tests/Client/MessageClientTest.php
@@ -55,10 +55,10 @@ public function testBuildCurlPostFields(): void
         $client = new MessageClient();
         $curlPostFields = $client->buildCurlPostFields($notification);
 
-        $this->assertIsArray($curlPostFields);
         $this->assertSame('cccc3333CCCC3333dddd4444DDDD44', $curlPostFields['token']);
         $this->assertSame('aaaa1111AAAA1111bbbb2222BBBB22', $curlPostFields['user']);
         $this->assertSame('This is a test message', $curlPostFields['message']);
+        $this->assertSame($message->getTimestamp(), $curlPostFields['timestamp']);
         $this->assertSame('ios,android', $curlPostFields['device']);
         $this->assertSame('This is a title of the message', $curlPostFields['title']);
         $this->assertSame('https://www.example.com', $curlPostFields['url']);