Invalid syntax for integer

hi ,

what is wrong with this if statement in this function

here distance variable is of type integer , this function has been created successfully but when i execute this function it gives me this error ,

ERROR:  invalid input syntax for integer: ""
LINE 1: SELECT distance =''
                                ^
QUERY:  SELECT distance =''
CONTEXT:  PL/pgSQL function "getsitesbytfid" line 6 at IF

********** Error **********

ERROR: invalid input syntax for integer: ""
SQL state: 22P02
Context: PL/pgSQL function "getsitesbytfid" line 6 at IF