writing/calling a function

I've written the following function the check whether or not a required field is empty upon form submission:

Code:

---

    $.fn.checkRequired = function(){
            return this.each(function(){
                        if (this.val()==''){
                                $("<div class='error'>some error message</div>").insertAfter('this');
                        };
            });
    };

---

I then check on submission like this:

Code:

---

                    $('form').submit(function(){
                            return $('form.required').checkRequired();       
                    });

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I'm not sure if I'm not calling the function correctly or if the function has problems (or both). Any ideas on how I might do this?