Case stmt in SQL

I have a UDF (User Defined function) that takes a JDE julian date and
converts it into Gregorian date. In the table, some of rows julian date is
zero, so I'm using case statment to exculde them. But when I run it, my UDF
is still being called for zero dates and casuing an exception..

SELECT case when
       ALEFTB =  0 then current_date ELSE julian(int(ALEFTB))
        END
FROM f0116

Question is, why is julian UDF being called when ALEFTB is equal to zero?

Thanks, Wes