RequiredField validator produces an unexpected error has occured error...

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SharePoint

When I attempt to add the requiredfield control to a SharePoint 2007 custom form connected to a list attempting to require a selection in a lookup field populated drop down list I receive the "an unexpected error has occured" error. Unfortunately there are no further details provided.

I'm guessing that it has something to do with the controltovalidate value but I've tried the SharePoint:FormField id of ff1{$Pos} and a number of other variations with no success. Maybe I'm missing something else altogether.

I know that the form works without the validator because as soon as I delete it from the page it loads just fine.

Any ideas?

Ralph