Returning a Stream in an ObjectScript Custom SQL Function

I'm attempting to extract some very large messages from Interoperability/Ensemble, and I'm just not getting something right ...

The closest I've come to getting something useful is this:

ClassMethod GetHL7Msg(pId As %String) As %Stream.TmpCharacter [ SqlName = GetMsg, SqlProc ]
{
    Set tHl7 = ##class(EnsLib.HL7.Message).%OpenId(pId,,.tSC)
    Throw:$$$ISERR(tSC) ..GetErr(-400, "HL7 Message with ID "_pId_" Not Found.")
    Set tMsg = ##class(%Stream.TmpCharacter).%New()
    Set tSC = tHl7.OutputToLibraryStream(.tMsg)
    Do tHl7.%Close()
    Return tMsg
}

When I call this via an ODBC query:

SELECT ID, SourceConfigName, TargetConfigName, HICG.GetMsg(MessageBodyId) As Message FROM Ens.MessageHeader 
  WHERE 
    MessageBodyClassName = 'EnsLib.HL7.Message' AND
    TargetConfigName = 'OptumMdmOut'
  ORDER BY ID ASC

I get a single, properly formed row back (there should be 1,000), and an error in the xDBC Errors log in IRIS:

Any thoughts?