How to get rid of 'undefined' is null or not an object error?

I currently have a page with 2 radio buttons to determine if the user is a returning customer or a new customer.  When the page loads it displays the radio buttons with the returning customer selected.  Below it is a login control that is contained in a separate user control.  If the user wishes to then login the page works fine but if they try to select the "new customer" radio button it does nothing and comes up with the following error:  'undefined' is null or not an object.  What should happen when the radio button is clicked is that the login control should be hidden and panel containing the new user fields should appear.