[AccessD] Debugging problem

I'm doing some work on a colleague's app and coming across something very
weird. In a Form_Open event, there's a line to test the value of Faculty_ID
and then make a decision based on it. The app crashes there and the error
message is:
	Run time error 2427
	You entered an expression that has no value

Yet when I ask IsNull(Me.Faculty_ID) the answer is False.

How can both things be true?

I've seen this happen before but it was long ago and I don't remember how I
fixed it. 

Ideas?

TIA,
Arthur