My query does not return what I think should be the Null Values

I have a very simple table. It has 12 fields, but I am only interested in GroupId, and dimension. dimension has what I believe to be some null values. when I run a simple query, both fields as output, and field dimension sorted ascending, I get my correct results. But when I run it with IS NULL as a criteria, I get and empty answer set.

see attached screen shots.



Any idea what I am missing?

Attached Files

Is null not working.doc (80.5 KB)

My query does not return what I think should be the Null Values