This was reported on discord a few minutes ago:

[00:12] Wikikinetic: Just encountered another glitch with item positioning:
[00:15] Wikikinetic: When I have a character with an item, it slots into their equipment slot just fine, but when I try to drag the tooltip to the place I want the item positioned on the character, dropping the tooltip to position causes this Lua error to appear:

I think we're assuming that StartMoving is going to assign a TOPLEFT point when called, which isn't guaranteed since it'll instead anchor to the nearest screen corner; apparently moving the window to the top left of the screen does indeed help resolve the issue.

Also I dunno what the heck is actually being dragged, so I'm just calling them "item tooltip markers".

Duplicate of #153 :3