Form validation during database Record Update (dreamweaver/php/sql)

I  am using the Dreamweaver Update Record Wizard to update my database records.  Basically, Dreamwevar creates for you an update form and then writes it to the database when you click the submit button on the form. 

I edited the update form to include onsubmit="return MyValidateFields(this);"  to validate my fields before writing them to the database. MyValidateFields() is just a JavaScript function I wrote to validate fields such as password, username, etc.  This method works very well with the Insert Record behavior,  but I am not getting it to work with the Update Record Behavior.  It looks as though when the submit button is clicked the MyValidateFields function is not even invoked. Am I doing something wrong, or is it because this onsubmit feature can not be used with the Update Record behavior? Any ideas?