Reverting a migration causes a string to be unserialized (which throws an error).
What was the error message?
Why would unserialize throw an error if a string is passed?
http://php.net/manual/en/function.unserialize.php says "mixed unserialize ( string $str )"
Do you mean "empty string"?
No, I mean a plain text string. unserialize only accepts a string of characters from the output of serialize or it throws an error.
I do not recall the exact error message.