[jQuery] Validation on a Modal form does not work

I have a modal form that I want to validate.
The modal is based on the simple modal plugin and the validation is
based on validation plugin.
The validation works well on a regular (non modal) form. ANy ideas?

 <script type="text/javascript">
            jQuery.validator.addMethod("zip", function(value, element)
{
                return this.optional(element) || value.match(/^((\d{5}-
\d{4})|(\d{5})|([a-z]\d[a-z]\s?\d[a-z]\d))$/i);
            }, "US or Canadian postal code only");
            $(document).ready(function(){
                getaddressBook();
                $("#addAddress").validate({
                    rules: {
                        emzip: {
                            required: true,
                            zip: true,
                        },
                    },
                });
            });
        </script>