Laravel validation Using Closures

Developing with Laravel 5.4 I am trying the example from Laravel documentation:

$validator = Validator::make($request->all(), [
'title' => [
    'required',
    'max:255',
    function($attribute, $value, $fail) {
        if ($value === 'foo') {
            return $fail($attribute.' is invalid.');
        }
    },
],

]);

I am getting this error:

Object of class Closure could not be converted to string

Why?

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