diff --git a/src/Mutator/Extensions/MBString.php b/src/Mutator/Extensions/MBString.php
index d332870b1..d8035ec70 100644
--- a/src/Mutator/Extensions/MBString.php
+++ b/src/Mutator/Extensions/MBString.php
@@ -103,7 +103,7 @@ private function setupConverters(array $functionsMap): void
             'mb_convert_case' => $this->mapConvertCase(),
         ];
 
-        $functionsToRemove = \array_filter($functionsMap, function ($isOn) {
+        $functionsToRemove = \array_filter($functionsMap, static function ($isOn) {
             return !$isOn;
         });