Run time error '2001'

[Name]

I have a form with a subform. I first built the form and
then the subform which has an event procedure in the
before update. Everything was working fine until I
wanted to change (or delete) a field in the main form.
Now when I add data (in testing) in the main form and tab
to the sub form I get the message "run time error '2001'
you cancelled previous operation".

I cannot figure out what's wrong. Can anyone help?

Thanks!
Nancy