I am assuming you have a query for the form recordsource, [quote]" however I created a calculation in a query using IIF"[Quote]
A ZLS is short for Zero Length String, which is exactly what it says a string of zero length, that 'looks like' it could be Null.
So I am saying inspect your data.
Alternatively do what I always did, walk through your code line by line, inpecting values.
See debugging tips in my signature.