This is the kind of thing that makes developing software take twice as long as it should.
So I run the function to test if a certain value is NULL (i.e. empty) and this is the error message I get:
"Data is Null. This method or property cannot be called on Null values."
Got that: the function to check if something is a Null value or not doesn't work if the value is Null, which you can't tell unless you run the function.