diff --git a/src/Collect/Support/Traits/EnumeratesValues.php b/src/Collect/Support/Traits/EnumeratesValues.php
index 4fe41f7..b29ab2f 100644
--- a/src/Collect/Support/Traits/EnumeratesValues.php
+++ b/src/Collect/Support/Traits/EnumeratesValues.php
@@ -159,7 +159,7 @@ public static function empty()
      * @param  (callable(int): TTimesValue)|null  $callback
      * @return static<int, TTimesValue>
      */
-    public static function times($number, callable $callback = null)
+    public static function times($number, ?callable $callback = null)
     {
         if ($number < 1) {
             return new static;
@@ -479,7 +479,7 @@ public function sum($callback = null)
      * @param  (callable($this): TWhenEmptyReturnType)|null  $default
      * @return $this|TWhenEmptyReturnType
      */
-    public function whenEmpty(callable $callback, callable $default = null)
+    public function whenEmpty(callable $callback, ?callable $default = null)
     {
         return $this->when($this->isEmpty(), $callback, $default);
     }
@@ -493,7 +493,7 @@ public function whenEmpty(callable $callback, callable $default = null)
      * @param  (callable($this): TWhenNotEmptyReturnType)|null  $default
      * @return $this|TWhenNotEmptyReturnType
      */
-    public function whenNotEmpty(callable $callback, callable $default = null)
+    public function whenNotEmpty(callable $callback, ?callable $default = null)
     {
         return $this->when($this->isNotEmpty(), $callback, $default);
     }
@@ -507,7 +507,7 @@ public function whenNotEmpty(callable $callback, callable $default = null)
      * @param  (callable($this): TUnlessEmptyReturnType)|null  $default
      * @return $this|TUnlessEmptyReturnType
      */
-    public function unlessEmpty(callable $callback, callable $default = null)
+    public function unlessEmpty(callable $callback, ?callable $default = null)
     {
         return $this->whenNotEmpty($callback, $default);
     }
@@ -521,7 +521,7 @@ public function unlessEmpty(callable $callback, callable $default = null)
      * @param  (callable($this): TUnlessNotEmptyReturnType)|null  $default
      * @return $this|TUnlessNotEmptyReturnType
      */
-    public function unlessNotEmpty(callable $callback, callable $default = null)
+    public function unlessNotEmpty(callable $callback, ?callable $default = null)
     {
         return $this->whenEmpty($callback, $default);
     }