mewebstudio/captcha wrong captcha in validation

I know this topic is existing, but no one could solve my problem.
I am using the library mewebstudio/captcha, everything is fine, but when I submit, it tells me always that the captcha is wrong.

My validation rules:

'captcha' => 'required|captcha'

In my html form:

<?= captcha_img(); ?>
<input type="text" name="captcha">

I am using php5.6, Laravel 5.1 and mews/captcha 2.2

via Chebli Mohamed