Help with formula. Keep getting error

In Crystal Reports, in selection formula, I keep getting the error:

The Result of selection formula must be a boolean

This is my formula:
if {?loc}='10135' then '10135,99999' else '{?loc},99998'

I am trying to have the user only enter 10135 and want the report to automatically add 99999 to the paramater selection. How can I achieve this? How do I get rid of the error.