Create Function Called On Null Input

I have created a function using:

CREATE FUNCTION MINOF (SMALLINT, SMALLINT, SMALLINT,
                       SMALLINT, SMALLINT, SMALLINT)
RETURNS SMALLINT
LANGUAGE RPGLE
PARAMETER STYLE GENERAL WITH NULLS
SPECIFIC MINOF NOT DETERMINISTIC
NO SQL CALLED ON NULL INPUT
EXTERNAL ACTION
FENCED
EXTERNAL NAME 'MAALIB/MINOF(MINOF)'

It's purpose is to simply return the minimum of the 6 input parameters.
The RPG code is trivial.  Here is the prototype:

D MINOF           PR             5I 0
D  A                             5I 0
D  B                             5I 0
D  C                             5I 0
D  D                             5I 0
D  E                             5I 0
D  F                             5I 0
D  INPIND                        5I 0 DIM(6)
D  RESULTIND                     5I 0

If parameter "A" is null then the UDF is not called.  I was able to
determine this, because I hard coded the return value to a number and the
result set returned a null (instead of the number) when "A" was null.  As
soon as I wrap the first parameter in "SMALLINT(IFNULL ( A ,   1235))",
everything works.  This fix doesn't work if I put it on the second
parameter, only the first.  Is there something I am missing?

Regards,

Mark