I created a Master ID for the purpose of matching between identities between two distinct tables. I did this by creating a Primary Master table and created subsequently a Master ID foreign key in the two tables I started with - Patrons and Donors. However, not every patron nor every donor has a match.

Problem is, I get an error when I try to enforce the integrity - it tells me there may be values that don't match to the Master table. But this is not the case.

I do, however, have null values in the foreign key in the two original tables. Is this a problem ? If so, do I simply designate the key as accepting Null values? Is there another possibility of causing this error?
Thanks, Leon