Darn null string issues

Ok, ongoing with my project here. I current have the seating list also create a phone list (to our current standards). The problem is I use this expression to output the extensions:
IIf(Len([Extension])>5,[Extension],"["+[Floor]+"-"+[Column]+"] "+[Extension])

which would generally come out like:

Lastname, Firstname [1-A1] 12345
or
Lastname, firstname 555-555-5555
(begining part is another text field, but you get the point)

I do this because there are some staff that are at site, but need a listed number. My problem is that there are other staff members that have ext. but no desks assigned (as yet, or ever). What happens is that if floor or column are Null(empty I guess, I tried testing for Null) nothing comes out in this text box.

In anycase, when I imported the data, in some of these cases both the Floor and Column were Null. This may be a field format issue, but I cannot get a value of " " to stay. I have set the Default Value to " " and this fixes the issue for later, but I don't want to have to go back and remove and recreate all those records. Also in future, I need to make sure that when the admin staff enter new employees, it does not break the code. Any ideas?

Jumped the gun on this. Testing for Null uses "Is Null" not "= Null". Lesson learned