partcombine discards quarter rests, why?

[Kenneth Wolcott]

Hi;

  I'm using \partcombine to merge two bass lines, one is a dotted half
note and the other bass one is a quarter rest and a half note.  The
upper voice has the quarter rest.  The quarter rest is NOT engraved.
The lower voice, having the dotted half note,is a fifth lower so there
is no collision, or shouldn't be.

Thanks,
Ken