In Access a boolean is two-valued: True/False. The value is stored in a bit, thus you really have a 1/0. I expect a 1 means true and a 0 means false, but couldn't really say for sure. Anyway, what I can say is that access consistently represents False as 0 and everything else as -1 or True. Conversely, access treats 0 as false and any other value as true.
I don't know paradox, but if you can create a view or query of your table over there, transforms the nil to a false, then you may be able to link to that query instead of the base table.