I think something like this should work. What error(s) are you getting? (I assume the use of $rowChecker in the last line is a transcription error.) My experience is that when I'm trying to check custom evaluators I end up having to put in a collection of warn() commands in the checker to see what different values are as the checker processes the answer.
The other thing that I often have to check is what type of object the checker is seeing by echoing out the ref() value of the object.
I don't know if that's useful, but hopefully it's not off base.