Error while creating eval expression for calculated fields in data models

I have a data model and defined about 5 fields. But one of the fields doesnt always have a value. I want it to show as "null" when there is no value in the log. So for this particular field, I created it as a calculated field(systemid) and gave an eval expression **systemid=if(isnull(systemid),"NULL",systemid)**. Now I get the error "Error in 'eval' command: Fields cannot be assigned a boolean result. Instead, try if([bool expr], [expr], [expr])."