writing/calling a function

[staypuffinpc]

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

    $.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:

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

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?