aegir migrate error

Loading apache driver for the http service
Loading mysql driver for the db service
Loaded alias @platform_drupal723 from file /var/aegir/.drush/platform_drupal723.alias.drushrc.php
WD php: PDOException: SQLSTATE[42000]: Syntax error or access violation: 1071 Specified key was too long; max key length is 1000 bytes: ALTER TABLE {sessions} ADD PRIMARY KEY (`sid`, `ssid`); Array ( ) in db_add_primary_key() (line 2891 of /var/aegir/platforms/drupal-7.23/includes/database/database.inc).