Jupgrade [migrating undefined]

The last week or so I've been working on updating and migrating a Joomla! site. Going from Joomla! 1.5.26 to 2.5 seemed to be smart upgrade. Unfortunately I ran into some problems when trying to use Jupgrade to migrate the database. I also got some problems with some outdated components but I got most of the stuff working.
The biggest problem I encountered was the "migrating undefined" problem. Luckily I found an easy, but perhaps ugly, solution to this problem.