Growth Calculation giving wrong Value

[cognos05]

Hi,
I am using cognos DImensional Package.

I have column with Prior Ytd, YTD, YTD change and Growth.

Growth is a calculated member and has the expression

if([Prior YTD]=0) Then (0)
Else
([YTD Change]/[Prior YTD])


But my result when Prior Ytd is zero is -1662,433.33% and not zero .


YTD Prior YTD  YTDChange  Growth
4987  0               4987           -1662,433.33%

I have set the Growth property in Data Format to % and Decimal point is 2

I am not sure why its putting some value when PrioirYTD is 0 .

Thanks,
Nithya