When you don't specify the FIRST opiton, it's a CAN-FIND UNIQUE.
That is supposed to fail, when there is a second record matching the criteria. I guess that's happening.
You should never use the FIND without FIRST (or LAST) when there is no UNIQUE index supporting the search. The AVM needs to search for the FIRST record and then also for a second record, to know if there's only a single record. That's a performance issue.