MRTG error on Debian Squeeze

The error:

rogue:~# mrtg /etc/mrtg.cfg
2014-04-28 16:32:17: WARNING: Can not determine ifNumber for public@127.0.0.1:     ref: 'Name'     key: 'eth0'
2014-04-28 16:32:17: WARNING: Can not determine ifNumber for public@127.0.0.1:     ref: 'Name'     key: 'eth1'
2014-04-28 16:32:17: ERROR: Target[127.0.0.1_eth0][_IN_] ' $target->[0]{$mode} ' did not eval into defined data
2014-04-28 16:32:17: ERROR: Target[127.0.0.1_eth0][_OUT_] ' $target->[0]{$mode} ' did not eval into defined data
2014-04-28 16:32:17: ERROR: Target[127.0.0.1_eth1][_IN_] ' $target->[1]{$mode} ' did not eval into defined data
2014-04-28 16:32:17: ERROR: Target[127.0.0.1_eth1][_OUT_] ' $target->[1]{$mode} ' did not eval into defined data

How to fix it:

# cd /var/lib/mrtg
# mv _etc_mrtg.cfg _etc_mrtg.cfg.old

Now run again mrtg /etc/mrtg.cfg - the error is gone.