Can't call method "id" on an undefined value

I got the following error message trying to use the Movable Type admin interface while upgrading from Movable Type 2.51 to Movable Type 2.63:

It turned out that I'd only copied the files, and not yet run the upgrade scripts. In my case the script was mt-upgrade.26.cgi, but check the upgrade documentation for the version you are upgrading to. The upgrade scripts re-jig the database schema a bit. Run them.

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