Return the string as-is, as a previous line does

src/Serializer/AbstractSerializer.php

@@ -291,7 +291,7 @@ protected function serializeCallable($callable): string
         // Prefer a string representation for simple strings that might just
         // happen to be callable, such as "file".
         if (\is_string($callable) && strpos($callable, '::') === false && strpos($callable, '_') === false) {
-            return $this->serializeString($callable);
+            return $callable;
         }
 
         try {