QF: own rule with callback and 2 parameters

Hi all, I've created an own rule for my form. This rule should use a callback function and a second parameter should be passed to that function. If this parameter is an integer, the variable inside the function is NULL but if it is a string, I get the correct value within the function. Snippet from my code: function qfRuleEmailExists($EMail, $IDUser) { // check mail address in DB // $IDUser is NULL, if passed as integer } $form->registerRule('EmailExists', 'callback', 'qfRuleEmailExists'); $form->addRule('E-Mail', 'Die E-Mail Adresse existiert schon!', 'EmailExists', $IDUser); Bug or Feature? Any suggestions? Thanks in advance, Peter