Unknown Error on Commit

Does anyone have any thoughts on receiving "Unknown Error" from FileMaker on a commit() command?

I have one page where a user can click a link and "sign up" on a slot. It works, using setField() and commit().

I have another page where a user can click on his own name to "unsign" from the slot. It does not work, also using setField() and commit(). I can't for the life of me see any difference in the commit commands. This is code from the page that does NOT work:

$findCommand = $fm->newFindCommand($layoutName);



$findCommand->addFindCriterion('n_SlotID', $slotID );



$result = $findCommand->execute();



$records = $result->getRecords(); 

$currentRecord = $records[0];



$currentRecord->setField('tUser_ID', $nullText);

$currentRecord->commit();