Possibly more than one problem with this. The first that comes to my mind is that you cannot say If A is something or Null. It has to be If A is something OR A IS Null...
So you certainly have a 2 part problem; the second being that IsNull is for VBA and Is Null is for sql.
EDIT:
Should add that nothing is ever = to Null. Null is 'unknown' thus it cannot be compared to anything in any fashion.
I'd also suggest you read up on how to name things. Your expression contains several examples of things you should not do.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.