javascript error message is not showing up
===

I am trying to make an error if the textbox (First_Name) is left empty. For example, if the firstname field is left empty, the user will receive the following error: "Please enter your first name" Nothing happens when I click the submit button after leaving the field blank though...

    <HTML>
    <HEAD>
    <TITLE>this is a test page</TITLE>
    <script language="javascript" type="text/javascript">
    <!--
    function CheckForm()
    {
      var formObj = document.getElementById("Data");
      var firstname = formObj.FIRST_Name.value;
    
    	if(notEmpty(firstname, "Please enter your first name")){						
    							
    	return true;}
    	
    	return false;
    	}
    
    function notEmpty(elem, helperMsg){
    	if(elem.value.length == 0){
    		alert(helperMsg);
    		elem.focus(); // set the focus to this input
    		return false;
    	}
    	return true;
    }
    
    </script>
    </HEAD>
    <BODY>
    <div style="background: #CCCC99">
    <HR><FORM id="Data" onsubmit="return CheckForm()" >
    <P>First Name: <input type=text name=FIRST_Name maxlength=15 size=15>
    </P>
    <input type=submit value="Submit Products Registration Form" style="width: 220px"><input type=reset value="Reset">
    </form>
    </div>
    </BODY>
    </HTML>