A string is required here (If Else Statement) in Crystal 2008

Hi I am getting an error 'A string is required here' when trying the following in Crystal 2008:

If {InvPrice.SellingPrice} = 0 then "0" else

({InvPrice.SellingPrice}-ccur({?Pm-Documents/Document/Det

Can someone please give me a clue on how to resolve this?

Thanks

Dom