diff --git a/Section.php b/Section.php
index 2800367..2a34db1 100644
--- a/Section.php
+++ b/Section.php
@@ -36,7 +36,7 @@ class Section
      * @param float|null $origin        Set the origin of the events in this section, use null to set their origin to their start time
      * @param bool       $morePrecision If true, time is stored as float to keep the original microsecond precision
      */
-    public function __construct(float $origin = null, bool $morePrecision = false)
+    public function __construct(?float $origin = null, bool $morePrecision = false)
     {
         $this->origin = $origin;
         $this->morePrecision = $morePrecision;
diff --git a/Stopwatch.php b/Stopwatch.php
index 139d665..538898f 100644
--- a/Stopwatch.php
+++ b/Stopwatch.php
@@ -61,7 +61,7 @@ public function getSections(): array
      *
      * @throws \LogicException When the section to re-open is not reachable
      */
-    public function openSection(string $id = null)
+    public function openSection(?string $id = null)
     {
         $current = end($this->activeSections);
 
@@ -100,7 +100,7 @@ public function stopSection(string $id)
     /**
      * Starts an event.
      */
-    public function start(string $name, string $category = null): StopwatchEvent
+    public function start(string $name, ?string $category = null): StopwatchEvent
     {
         return end($this->activeSections)->startEvent($name, $category);
     }
diff --git a/StopwatchEvent.php b/StopwatchEvent.php
index 3bca2fd..1a85d80 100644
--- a/StopwatchEvent.php
+++ b/StopwatchEvent.php
@@ -42,7 +42,7 @@ class StopwatchEvent
      *
      * @throws \InvalidArgumentException When the raw time is not valid
      */
-    public function __construct(float $origin, string $category = null, bool $morePrecision = false, string $name = null)
+    public function __construct(float $origin, ?string $category = null, bool $morePrecision = false, ?string $name = null)
     {
         $this->origin = $this->formatTime($origin);
         $this->category = \is_string($category) ? $category : 'default';