Help with IF Fomula

I have this fomula set for this condition. But it's not giving me right result. When Prior Amount is 0 I want it to show Prior Amount = 0 and not Blank or Zero.

=IF Sum([Prior Amount]) = 0 Then "Prior Amount = 0" Else Sum([Difference])/Sum([Prior Amount]).

All help appreciated in getting this to show "Prior Amount = 0" will be helpful.

Pls note I used the Sum to avoid multi value and it works but I need the Prior amount message to work. Thanks for all your help - much appreciated.