pkgdb failures

[Yuri]

While doing portupgrade I am sporadically getting the error like below.
After I ran 'pkgdb -fu' few times and this cured tye problem. But then after
a while of portupgrade run I now get the error:

>pkgdb -fu
>--->  Updating the pkgdb
>[Rebuilding the pkgdb <format:bdb_btree> in /var/db/pkg ...
/var/db/pkg/pkgdb.db: >unexpected file type or format -- Invalid argument:
Cannot update the pkgdb!]

man pkgdb says:
BUGS
     Sometimes a database may get corrupt, and the pkgtools commands may abort
     with a segmentation fault.  In such cases, run ``pkgdb -fu'' to rebuild
     the database, and the problems should go away.

So what should I do now to upgrade ports?

Yuri

-- error log during portupgrade --
/var/db/pkg/pkgdb.db: unexpected file type or format -- Invalid argument
** Command failed [exit code 1]: /usr/local/sbin/pkgdb -aFOQ
--->  Skipping 'net/openldap23-client'
--->  Reinstalling 'tcl-8.4.16,1' (lang/tcl84)