jqeury validate not working after add radiobutton

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ASP.NET

HTML

jQuery

validation

, +

im using jquery.validate.min.js

<script>
        $(document).ready(function () {

            $("#form1").validate({               
                onsubmit: false,
                validClass: "has-success"
            });

            $('.validationGroup .causesValidation').click(Validate);
        });

        function Validate(evt) {
            var $group = $(this).parents('.validationGroup');
            var isValid = true;
            $group.find(':input').each(function (i, item) {
                if (!$(item).valid()) {
                    $(this).closest('.validationGroup').addClass('has-error');
                    isValid = false;
                }
            });

            if (!isValid) evt.preventDefault();
        }
    </script>

the code is working until i add the radiobutton code into the form

<div class="btn-group" data-toggle="buttons">
<label class="btn btn-default active">
<input type="radio" value="M" />Male
</label>
<label class="btn btn-default">
<input type="radio" value="F" />Female
</label>
</div>

now, whenever i click the submit button, the form call a postback, which it shouldn't. it suppose to check for the validation first then only call a post back. any idea where is the error?