Operational Descriptors

Here's a strange one...

I'm using the OPDESC keyword on a procedure.
The 3rd parameter of that procedure is defined as follows (well
actually, here's the entire proc interface):

     D GetNextEntry    PI            10I 0 OPDESC
     D  szUserSpace                  20A   Const
     D  nRefPos                      10I 0 
     D  szEntry                    4096A   OPTIONS(*VARSIZE)

Upon entering the procedure, I call CEEDOD, or CEEGSI.
I do that to extract the length of parameter 3.

Upon calling either of these APIs, I get a "CEE0502 Missing operational
descriptor" message.

Now here's the strange part:
If I call this procedure from within another procedure, I don't get the
error and the APIs work fine. But if I call the exact same procedure
from the mainline calcs in a program, I get the above mentioned error.

Is this a bug, has anyone else come across this?

OS/400 V4R5.

Bob Cozzi