I'm getting a fatal error message in the inbox page:

I'm getting a fatal error message in the inbox page: http://4bc.907.myftpupload.com/inbox/

Fatal error: Call to undefined method MM_Conversation_Model::fetch_array() in /home/content/p3pnexwpnas08_data02/48/2511548/html/wp-content/plugins/private-messaging/app/models/mm-conversation-model.php on line 73

Can you help me to fix this? Thanks