SQL UDF From RPG Program

Trying to write an SQL UDF using RPG. Usually I put them in a service
program, but this time I was trying to do it with a regular program.

Here is the interface for the program. The function accepts two zip codes
and returns the distance between them.

D SqlGetZipD pi
D pZipCode1 5a
D pZipCode2 5a
D pRetDist 8f
D pInpInd 5i 0
D pOutInd 5i 0
D pOutSqlState 5a
D pInpFuncName 139a varying
D pInpSpecName 128a varying
D pOutDiagTxt 70a varying

Here is the function code:

CREATE FUNCTION DMIPGMS/GETZIPDIST ( CHAR(5), CHAR(5) )
RETURNS DOUBLE
LANGUAGE RPGLE
PARAMETER STYLE SQL
DETERMINISTIC
PROGRAM TYPE MAIN
NO SQL
RETURNS NULL ON NULL INPUT
EXTERNAL NAME 'DMIPGMS/SQLGETZIPD';


Not sure what I am doing wrong. Any help would be appreciated.

Thanks,
Matt