I'm beginning to suspect so, as there is no other logical alternative. I've not got access to one [sharde drive], myself, but perhaps someone who has will know. [The help file is silent on the issue.] I've saved it to a 2nd hard drive on my machine, closed out the file, and the formula works fine.
Wookie -- If the formula finds Feb when the WB is closed, the problem is not a system problem, but a data problem of some sort. Double check the Month setting -- is it a computed cell? Are any of the values in the 1st row - especially E4 - computed/subject to change?