Merged [6468] and [6469] from ejimenez@yaco.es:

Now incumbent can be None in positions.
Fixes ietf-tools#1156.
 - Legacy-Id: 6470
Note: SVN reference [6468] has been migrated to Git commit d4e37ac

Note: SVN reference [6469] has been migrated to Git commit 0a31572

Author: levkowetz