CiviMail fatal error in 4.6.4
There was a bug in CiviCRM 4.6.4 that triggered the same error 'A fatal error was triggered: is not of the type Int' when trying to rebuild the smart group cache so this could be related, especially if U used smart groups for your maling.
I would suggest upgrading to 4.6.6 on a copy installation to see if that resolves the issue. Since this is not a real answer to your question I wanted to post this as a comment but I don't have sufficient reputation points to do that. I myself upgraded to 4.6.6 after experiencing too many problems with the 4.6.4 upgrade.