Why debug is not properly working in jquery validation plugin?

Why debug is not properly working in jquery validation plugin?

 when I used debug true in this script warning show in console that : Uncaught TypeError: form.submit is not a function at a.validator.submitHandler()
and When i do not use debug method all are working fine..
help me guzz how to use debug true method properly...
i am bigginer in Coding...
my Code Are below:  

HTML CODE: 

  3. <form action="test.php" method="GET" role="form" id="signupForm">
  4.     <legend>Demo Form Validation</legend>
  5.       <div class="form-group">
  6.         <input type="text" class="form-control" autocomplete="off" id="fname" name="fname" placeholder="Firstname">
  7.       </div>
  8.       <div class="form-group">
  9.         <input type="text" class="form-control" autocomplete="off" id="lname" name="lname" placeholder="Lastname">
  10.       </div>
  11.       <div class="form-group">
  12.         <input type="text" class="form-control" autocomplete="off" id="mobile" name="mobile" placeholder="Mobile Number">
  13.       </div>
  14.       <div class="form-group">
  15.         <input type="text" class="form-control" autocomplete="off" id="email" name="email" placeholder="Email">
  16.       </div>
  17.       <div class="form-group">
  18.         <input type="text" class="form-control" id="password" autocomplete="off" name="password" placeholder="Password">
  19.       </div>
  20.       <div class="form-group">
  21.         <input type="text" class="form-control" id="repassword" autocomplete="off" name="repassword" placeholder="Repeat password">
  22.       </div> 
  23.       <button type="submit" id="submit" name="submit" class="btn btn-primary">Submit</button>
</form>


JQUERY Code:  

$(function() {

    $.validator.addMethod("mobilevalid",function function_name(value,element) {
        return this.optional(element) || /^\d{10}$/.test(value);
    }, "Plese enter a valid mobile number.");

    jQuery.validator.addMethod('strongPassword', function (value, element) {
    return this.optional(element)
      || value.length >= 6
      && /\d/.test(value)
      && /[a-z]/i.test(value);
  }, 'Your password must be at least 6 characters long and contain at least one number and one char.');

        $("#signupForm").validate({
        debug: true,
        focusInvalid: false,
        rules: {
            fname: {
                required: true,
                minlength: 3,
                lettersonly: true           
            },
            lname: {
                required: true,
                minlength: 3,
                lettersonly: true
                
            },
            mobile: {
                required: true,
                mobilevalid: true
            },
            email: {
                required: true,
                email: true
            },
            password: {
                required: true,
                strongPassword: true
            },
            repassword: {
                equalTo: "#password"
            }
        },
        messages: {
            fname: {
                required: "Firstname field cannot be empty.",
                lettersonly: "Only letter are allowed and whispace are not allowed.",
            },
            lname: {
                required: "lastname field cannot be empty.",
                lettersonly: "Only letter are allowed and whispace are not allowed.",
            },
            email: {
                required: "email field cannot be empty.",
                email: "This is not a valid email"
            },
            repassword:{
                equalTo: "Password doesn't match."
            }
        },
        submitHandler: function(form) {  

                               form.submit(); 

                    }
        
    });
    

    
});