TinyMCE & Fancybox – editor won’t work on second view

I’ve added TinyMCE to my project, and am using it on a text area which pops up in a fancybox. The first time I action it, it works fine, but if I then close it and try to open it again, it doesn’t let me type in the box. It looks okay, just that the text area is kind of greyed out, and won’t accept input. If I click any of the buttons (bold, italic, justify, font selection etc.), the console gives the error “j is null”.