This is one of those ‘OMG Eriky, you saved my ass!’ posts.

You probably came here after searching for this error. Before you try anything else, check if your /tmp directory exists and if it has the right permissions. If not, create it and do a “chmod 777 /tmp”. MySQL will give you the weirdest errors if it can not use the /tmp folder.

If this does not help, check your /etc/mysql/my.cnf. There could be a line somewhere, stating which directory to use as a temporary directory. Create that directory, or remove/change the line so MySQL uses your /tmp folder instead.