Here is the solution. When I use the function wizard to enter the
equation the result is this:

=IF(IMREAL(B31)=0,ATAN(IMAGINARY(B31)/0.000001),"ATAN(IMAGINARY(B31)/IMREAL(*B31)))"

When I take out the quotes like bj suggested (by directly editing the
cell formula) it works.

Looks like a bug in the function wizard. Agree?

thanks,
russ