Formula Error ( Past Due -Ageing)

Hi guys, Ive been trying to customize a report in Crystal but I got trouble in formula for Ageing by month. The formula that Im using is having an error. Can anyone help me fix it.

my formula is ...

if datediff("m",{@Last Day (DD)},{@Last Day (CD)}) = 0

then

{@Balance}

else

0

I have attachment. Thanks in Advance!