Create an SQL function

Okay, back to trying to create an SQL function (at V4R5M0).  I have a service program with an RPGLE module which has the following prototype:

D CENTER          PR          2000A   OPDESC          
D                             2000A   OPTIONS(*VARSIZE)

It's pretty basic.  It's used to center a string.  I've created the SQL function with the following statement:

create function utilities/center (varchar(2000))
returns char(2000)                             
external name 'UTILITIES/STRINGS(CENTER)'      
returns null on null input                     
Language RPGLE                                 
No SQL                                         
parameter style general                        
no external action                             

When I try to use the function, I receive the sql error: "Query cannot be run.  See lower level messages." and when I look at the lower level messages, I see: CEE0502 - "Missing operational descriptor.".

Anyone have any ideas what I'm doing wrong?  Has anyone created an SQL function over an RPG module in a service program?

Dave