Validating a field using SQL custom function

I have a uniqueness criteria for a particular number field, but the uniqueness is based on a combination of the number field with a state field (e.g. the number must be unique within all records for that state). I built a custom function to return a boolean if the number+state combination already exists in the database, and that CF uses a simple SQL query,

Issue is that the CF seems to work flawlessly when tested via Data Viewer, but when inserted as the field validation calculation, returns a validation failure for any value entered.  

Can anyone enlighten me as a to a limitation here I may not be aware of? My work around path is a field level trigger to a validation script, but do feel like I am missing something. 

Cheers,
Juz.