Adding days to a date field with lookup

Adding days to a date field with lookup

Hi

I have a potential order closing date (variable) that I would like to add a number of days (variable) to depending on the value in the field, this is so that I can calculate revenue date and amount. 

So far I have created the formula Adddate (${Potentials.Closing Date} ,${Potentials.Days} ,'DAY') but this returns Incorrect data type of argument.

Is there a way of adding the two variables together?

Guy