I'm attempting to add a new field, Narrators, and sidebox to the 'Book Product Type' so it will work better with Audio Books. I based everything I did on the Author field and it works great except when I have the Narrator sidebox select a narrator that has more than one book. Then I get an error message

1065 Query was empty
in:
[]

on the screen.

I've looked for where the narrator filter query is built and everything looks fine. I can't find where the query string is used nor where it could be set to a null or empty string.

Can anyone help out by pointing me in the right direction and/or tell me where to loo?