error during upgrading

[mrl586]

I got following error when I upgrading percona.
root@vps:~# apt-get upgrade
Luetaan pakettiluetteloita... Valmis
Muodostetaan riippuvuussuhteiden puu
Luetaan tilatiedot... Valmis
Nämä paketit päivitetään:
  ftp iptables libmysqlclient18 libselinux1 netbase percona-server-client percona-server-client-5.5 percona-server-common-5.5 percona-server-server percona-server-server-5.5
10 päivitetty, 0 uutta asennusta, 0 poistettavaa ja 0 päivittämätöntä.
Noudettavaa arkistoa 61,5 Mt.
Toiminnon jälkeen käytetään 20,5 k t lisää levytilaa.
Haluatko jatkaa [K/e]? k
Nouda:1 http://ftp.de.debian.org/debian/ wheezy/main libselinux1 amd64 2.0.98-1.1 [89,0 kB]
Nouda:2 http://ftp.de.debian.org/debian/ wheezy/main iptables amd64 1.4.11.1-2 [377 kB]
Nouda:3 http://ftp.de.debian.org/debian/ wheezy/main netbase all 4.46 [21,0 kB]
Nouda:4 http://ftp.de.debian.org/debian/ wheezy/main ftp amd64 0.17-25 [62,3 kB]
Nouda:5 http://repo.percona.com/apt/ squeeze/main libmysqlclient18 amd64 5.5.13-rel20.4-136.squeeze [4 502 kB]
Nouda:6 http://repo.percona.com/apt/ squeeze/main percona-server-common-5.5 all 5.5.13-rel20.4-136.squeeze [2 736 B]
Nouda:7 http://repo.percona.com/apt/ squeeze/main percona-server-client-5.5 amd64 5.5.13-rel20.4-136.squeeze [19,5 MB]
Nouda:8 http://repo.percona.com/apt/ squeeze/main percona-server-client all 5.5.13-rel20.4-136.squeeze [2 626 B]
Nouda:9 http://repo.percona.com/apt/ squeeze/main percona-server-server-5.5 amd64 5.5.13-rel20.4-136.squeeze [36,9 MB]
Nouda:10 http://repo.percona.com/apt/ squeeze/main percona-server-server all 5.5.13-rel20.4-136.squeeze [2 742 B]
Noudettiin 61,5 Mt ajassa 2min 7s (482 kt/s)
Esiräätälöidään paketteja...
(Reading database ... 44745 files and directories currently installed.)
Preparing to replace libselinux1 2.0.98-1+b1 (using .../libselinux1_2.0.98-1.1_amd64.deb) ...
Unpacking replacement libselinux1 ...
Setting up libselinux1 (2.0.98-1.1) ...
(Reading database ... 44745 files and directories currently installed.)
Preparing to replace iptables 1.4.10-1 (using .../iptables_1.4.11.1-2_amd64.deb) ...
Unpacking replacement iptables ...
Preparing to replace netbase 4.45 (using .../archives/netbase_4.46_all.deb) ...
Unpacking replacement netbase ...
Preparing to replace ftp 0.17-24 (using .../archives/ftp_0.17-25_amd64.deb) ...
Unpacking replacement ftp ...
Preparing to replace libmysqlclient18 5.5.12-rel20.3-118.squeeze (using .../libmysqlclient18_5.5.13-rel20.4-136.squeeze_amd64.deb) ...
Unpacking replacement libmysqlclient18 ...
Preparing to replace percona-server-common-5.5 5.5.12-rel20.3-118.squeeze (using .../percona-server-common-5.5_5.5.13-rel20.4-136.squeeze_all.deb) ...
Unpacking replacement percona-server-common-5.5 ...
Preparing to replace percona-server-client-5.5 5.5.12-rel20.3-118.squeeze (using .../percona-server-client-5.5_5.5.13-rel20.4-136.squeeze_amd64.deb) ...
Unpacking replacement percona-server-client-5.5 ...
Preparing to replace percona-server-client 5.5.12-rel20.3-118.squeeze (using .../percona-server-client_5.5.13-rel20.4-136.squeeze_all.deb) ...
Unpacking replacement percona-server-client ...
Preparing to replace percona-server-server-5.5 5.5.12-rel20.3-118.squeeze (using .../percona-server-server-5.5_5.5.13-rel20.4-136.squeeze_amd64.deb) ...
Stopping MySQL (Percona Server): mysqld.
Aborting downgrade from (at least) 5.5 to 5.1.
If are sure you want to downgrade to 5.1, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing /var/cache/apt/archives/percona-server-server-5.5_5.5.13-rel20.4-136.squeeze_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1
configured to not write apport reports
                                      Stopping MySQL (Percona Server): mysqld.
Starting MySQL (Percona Server) database server: mysqld . . . . . . . . . . . . . . failed!
invoke-rc.d: initscript mysql, action "start" failed.
dpkg: error while cleaning up:
 subprocess installed post-installation script returned error exit status 1
Preparing to replace percona-server-server 5.5.12-rel20.3-118.squeeze (using .../percona-server-server_5.5.13-rel20.4-136.squeeze_all.deb) ...
Unpacking replacement percona-server-server ...
Processing triggers for man-db ...
Errors were encountered while processing:
 /var/cache/apt/archives/percona-server-server-5.5_5.5.13-rel20.4-136.squeeze_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)