Syntax to search a checkbox field

Hi,

I'm trying to add a filter on a custom field that is defined as a checkbox.  I can't seem to get the results I'm expecting.

Here is the filter definition:

filters[1] = new nlobjSearchFilter('custrecord_upgrade', null, 'isnot', 'T');

I've also tried:
filters[1] = new nlobjSearchFilter('custrecord_upgrade', null, 'is', 'N');

Does anyone have ideas on how to filter on a checkbox value?

(note: this is just a snippet.  There is a filters[0] and the appropriate searchRecord.  The search works if I remove the checkbox entry).

Thanks,

Brian