get rid of the quotes the equation thinks you want text.
=IF(IMREAL(B31)=0,ATAN(IMAGINARY(B31)/0.000001),ATAN(IMAGINARY(B31)/IMREAL(B31)))
"Russ" wrote:

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