Validating if 'Parked By' Field is Empty

Hello all,

I'm trying to create a validation to prevent any user from directly posting a document, I'm doing this by "Checking" if field BKPF-PPNAM is empty, however if I enter the "Check" rule Parked by = '' the validation isn't triggering! I tried adding my user name there and it works, it's simply not working if the field is empty.

Anyway this can be managed?

Thanks for the help in advance.