diff --git a/src/Attributes/Validation/In.php b/src/Attributes/Validation/In.php
index 6040e6d7..2607b414 100644
--- a/src/Attributes/Validation/In.php
+++ b/src/Attributes/Validation/In.php
@@ -32,7 +32,7 @@ public function getRule(ValidationPath $path): object|string
         }
 
         $this->values = array_map(
-            fn (string|ExternalReference $value) => $this->normalizePossibleExternalReferenceParameter($value),
+            fn (mixed $value) => $this->normalizePossibleExternalReferenceParameter($value),
             Arr::flatten($this->values)
         );