Using qualified Ds in I/O

Hi all, maybe it's only my but I find this one annoyng:

Dcl-f mudbcos02l disk keyed usage(*input);

Dcl-Ds rDbcos extname('MUDBCOS00F':*ALL) qualified End-ds;

chain ('21903262') mudbcos02l rDbcos;

It's not working (RNF7595)

A workaround is:

Dcl-f mudbcos02l disk keyed usage(*input);

Dcl-Ds rDbcos extname('MUDBCOS00F':*ALL) qualified End-ds;
Dcl-Ds rCos likerec(dbcos);

chain ('21903262') mudbcos02l rCos;
eval-corr rDbcos = rCos;
if %nullind(rDbcos.Duedate);
%nullind(rDbCos.Duedate) = *off;
rDbCos.Duedate = %date();
EndIf;

Or (but max field's name length is 14 and will be prefixed in the whole
programm):

Dcl-f mudbcos02l disk keyed usage(*input) prefix('RS.');

Dcl-Ds RS extname('MUDBCOS00F':*ALL) qualified End-ds;

chain ('21903262') mudbcos02l;
if %nullind(RS.Duedate);
%nullind( RS.Duedate) = *off;
RS.Duedate = %date();
EndIf;

So my question is: why is it not possible to use a qualified Ds in I/O
operations?

TIA
--
Marco Facchinetti

Mr S.r.l.

Tel. 035 962885
Cel. 393 9620498

Skype: facchinettimarco