Cannot insert HTML code with img_assist

When I write a node, and try to insert an image with img_assist module, I can't insert the image with HTML code. Normally, the img_assist box should close when I click Insert, but instead, it goes blank, and nothing is inserted. If I use Filter tag, everything works as expected.

I tested it with a different theme (Twilight), and HTML code works with that one. I assume this is a Marinelli issue, and not a img_assist issue, but I could be wrong.