non-existent user "28"

Hi,
when I pg_dump my database, I get lines like:
GRANT SELECT on "tablename" to "28";

When I restore from the dump file, I get errors:
ERROR: aclparse: non-existent user "28"

The dump file does contain grants for the real users which exist in the
database, and I've checked pg_shadow, and there is no user 28.
I'm not sure how the database got in this state - would this happen when I
drop & recreate a user? Is there something I can do about this? I couldn't
see where in the system tables the permission info is stored.

Thanks,
Tamsin