diff --git a/src/Collections/Collection.php b/src/Collections/Collection.php
index f7fec10..cf58f32 100644
--- a/src/Collections/Collection.php
+++ b/src/Collections/Collection.php
@@ -136,14 +136,14 @@ class Collection extends ImmutableCollection implements ICollection {
      * @inheritDoc
      */
     public function sortSelf (Closure|IComparator|null $sorter = null): static {
-        uasort($this->elements, $sorter);
+        uasort($this->elements, self::_normalizeSorter($sorter));
         return $this;
     }
     /**
      * @inheritDoc
      */
     public function sortSelfByKey (Closure|IComparator $sorter): static {
-        uksort($this->elements, $sorter);
+        uksort($this->elements, self::_normalizeSorter($sorter));
         return $this;
     }
 }
\ No newline at end of file
diff --git a/src/Collections/ImmutableCollection.php b/src/Collections/ImmutableCollection.php
index 353a155..4dc51b9 100644
--- a/src/Collections/ImmutableCollection.php
+++ b/src/Collections/ImmutableCollection.php
@@ -286,19 +286,8 @@ class ImmutableCollection implements IImmutableCollection {
      * @inheritDoc
      */
     public function sort (Closure|IComparator|null $sorter = null): static {
-        if ($sorter === null) {
-            $sorter = function (IComparable $value1, IComparable $value2): int {
-                return $value1->compareTo($value2);
-            };
-        }
-        elseif ($sorter instanceof IComparator) {
-            $sorter = function ($value1, $value2) use ($sorter): int {
-                return $sorter->compare($value1, $value2);
-            };
-        }
-
         $elements = $this->toArray();
-        uasort($elements, $sorter);
+        uasort($elements, self::_normalizeSorter($sorter));
 
         return new static($elements);
     }
@@ -306,18 +295,43 @@ class ImmutableCollection implements IImmutableCollection {
      * @inheritDoc
      */
     public function sortByKey (Closure|IComparator $sorter): static {
-        if ($sorter instanceof IComparator) {
-            $sorter = function ($value1, $value2) use ($sorter): int {
-                return $sorter->compare($value1, $value2);
-            };
-        }
-
         $elements = $this->toArray();
-        uksort($elements, $sorter);
+        uksort($elements, self::_normalizeSorter($sorter));
 
         return new static($elements);
     }
 
+    /**
+     * Normalize a sorter method
+     *
+     * @param null|IComparator|Closure(TValue, TValue):int $sorter The sorting method ; Null if values are object implementing {@see IComparable}.
+     *                                                             <br>Return :
+     *                                                             <br>&lt; 0 if value1 is before value2
+     *                                                             <br>= 0 if value1 equals value2
+     *                                                             <br>&gt; 0 if value1 is after value2
+     *
+     * @return Closure(TValue, TValue):int The normalized sorter method
+     *                                     <br>Return :
+     *                                     <br>&lt; 0 if value1 is before value2
+     *                                     <br>= 0 if value1 equals value2
+     *                                     <br>&gt; 0 if value1 is after value2
+     */
+    public static function _normalizeSorter (Closure|IComparator|null $sorter): Closure {
+        if ($sorter === null) {
+            return function (IComparable $value1, IComparable $value2): int {
+                return $value1->compareTo($value2);
+            };
+        }
+        elseif ($sorter instanceof IComparator) {
+            return function ($value1, $value2) use ($sorter): int {
+                return $sorter->compare($value1, $value2);
+            };
+        }
+        else {
+            return $sorter;
+        }
+    }
+
     /**
      * @inheritDoc
      */
diff --git a/tests/test_class.php b/tests/test_class.php
index 837f4db..db96292 100644
--- a/tests/test_class.php
+++ b/tests/test_class.php
@@ -65,12 +65,15 @@ function getMeasures (): Collection {
     return $measures;
 }
 
+$measures = getMeasures();
+$measures->sortSelf();
+
 echo 'Measures :' . PHP_EOL;
-foreach (getMeasures()->sort() as $no => $measure) {
+foreach ($measures as $no => $measure) {
     echo "\t#" . $no . ' = ' . $measure->asString() . PHP_EOL;
 }
 
 echo 'Measures (reversed) :' . PHP_EOL;
-foreach (getMeasures()->sort(new MeasureReverseComparator()) as $no => $measure) {
+foreach ($measures->sort(new MeasureReverseComparator()) as $no => $measure) {
     echo "\t#" . $no . ' = ' . $measure->asString() . PHP_EOL;
 }
\ No newline at end of file