Error Message Display

hi I have a problem in Error message display

I select text from the UDF if it is null then I have to show some error message in Service call.

if(SELECT T0.[U_HPCaseID] FROM OSCL T0) is null

begin

set @error = -5500

set @error_message = 'please enter case Id'

end

I use this code for that field

but is adding success fully with out any error please help me to sort out this error

thanks and regards

D.Rajesh