Error displaying while working on interface

Hi 

Expression i used is:

if(todate(ri!vehicle.vehicleNextMaintenanceDate) <
todate(ri!vehicle.vehicleLastMaintenanceDate), “The next
maintenance date must be after the last maintenance date.”,
null)

Below message is the error i got while working on interface.

Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function 'if' [line 59]: Invalid number of parameters, function 'if' a minimum of 3 parameters (condition,true,false), passed 1.