Access 2007 error when entering data in form created in 2003

[Dirth Vader]

We recently upgraded from Office 2003 to 2007 and I have a user with an
Access 2003 database that isn't working. The DB uses forms and macros to
enter data. When entering data into a form, we get the following error if
the data exists in the database already:

The value violates the validation rule for the field or record

If we enter a unique value in the field, it works, but entering a value that
exists in the database does not work.

Using Access 2003 this works fine.

I've tried to convert the file to the Access 2007 format, but it does the
same thing.

I've narrowed it down a bit to the following macro entry:

Not IsNull([Forms]!(f-Main Entry Form]![Work Order Number]) Action =
StopMacro

If I turn on macro debuging, I see this turn TRUE, then I get the error when
I advance to the next step.

Anyone have any ideas?

Thanks!
Dirth