Upgrade database error

I am trying to upgrade our existing zimbra installation (ZCS 6.0.9) to ZCS 7.1.3 which is running on Ubuntu 8.04.3, however during the upgrade procedure it runs a database integrity check and I receive the following error:

Code:

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Database errors found.
/opt/zimbra/mysql/bin/mysqlcheck --defaults-file=/opt/zimbra/conf/my.cnf -S /opt/zimbra/db/mysql.sock -A -C -s -u root --password=xxxxxxxx --auto-repair
mboxgroup79.mail_item
 error    : Corrupt
 mboxgroup79.mail_item
 note    : The storage engine for the table doesn't support repair

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I have tried running through a procedure on the wiki Mysql Crash Recovery (alternate method) - Zimbra :: Wiki however when it gets to the section where you dump the individual errors out to /tmp/XX all the files are 0 size, so there doesn't appear to be anything for me to fix.

Anyone have any ideas? Can't seem to repair it but I don't know if it will cause nay problems if I just ignore it and carry on with the update.

login to mysql, use the db with the issue and try running: OPTIMIZE TABLE mail_item;