[SOLVED] Form validation in 3.1.7

The validation rule valid_email in CI's 3.1.7 form validation, doesn't work anymore.

PHP Code:

$this->form_validation->set_rules('email','E-mailadres','trim|required|valid_email|callback_email_check'); 


In 3.1.6 there's no problem.
In 3.1.7 I get the error "The field E-mail must contain a valid email address".

I'm using this rule in my sign-up form.
In the callback function I check if the email address already exists in the user table, or if there's a former sign-up request for the same e-mail address.