Nintex Form Validation

Good Afternoon,

                         We are using Nintext 2013 (Forms and Worfkow).  We have a simple javaScript validation that we added to a form to make sure a selection is made on a drop down list  "Make a Selection"   Option One   or  Option Two"  using an alert we can tell the script it working - but not sure what happens with the form validation after that.  

The problem is the Form validation worked before until we add the JavaScript - is there something we should do or know about after adding a Script to a Form - we are missing something but not sure exactly.   Any responce will be highly appreciated.

Thanks

Carol