Qualified DS parm not returned from service program

I'm either having a brain f*** today or forgetting something fundamental I think.

I'm passing a qualified DS as a parm to a service program. The service program populates this DS with up to 20 errors and returns.

I have stepped through the service program and confirmed it fills the DS properly, but when I get back to the calling program, the DS is blank. I do not have CONST or VALUE on the parm. It is the only parm that is changed in the service program.

"em" is the parm in question.

Prototype (copy member copied into both service program and caller) and procedure interface for the service program:

D EdtSngFlds PR
D cItemDS LikeDS(ItmMstDS)
D wItemDS LikeDS(ItmMstDS)
D em LikeDS(ErrMsgDS)
D PrtFrm 1A Const
D ReqAct 1A Const
D EffImm 1A Const
D PCNbr 2S 0 Const

P EdtSngFlds B Export
D EdtSngFlds PI
D cItemDS LikeDS(ItmMstDS)
D wItemDS LikeDS(ItmMstDS)
D em LikeDS(ErrMsgDS)
D PrtFrm 1A Const
D ReqAct 1A Const
D EffImm 1A Const
D PCNbr 2S 0 Const

Prototype and procedure interface to the calling program (note that "em" is a parm _into_ the calling program as well):

D IM01 PR ExtPgm('IM01')
D ITNBR 6A
D em LikeDS(ErrMsgDS)

D IM01_Init PR
D IM01_Unload PR

D IM01 PI
D pITNBR 6A
D em LikeDS(

Here's the definition of ErrMsgDS (copy member also copied into both service program and caller):

D @ErrMaxNbr C 20
D ErrMsgDS DS Qualified D ErrMsgID 7A Dim(@ErrMaxNbr) D ErrMsgTxt 75A Dim(@ErrMaxNbr)

Does it have anything to do with "em" being a parm _into_ the caller?

Thanks.