Merging sectors causing errors

Whenever I merge a large number of sectors (usually anything more than 2 or 3 sectors), I get an error when saving or going into 3d mode. So long as you save it, close it, and load it up again, it will say "removed unused sectors", and works again normally.

Just a bit annoying that it happens every time. I'm sure this is a well known bug for everyone, but just thought I'd mention it in case it isn't, I suppose.

Yes it's known. What's happening, is that there are sectors (like 3 and 4, if you merged 2, 3, and 4) which were written down, but don't exist/don't have any linedefs. So it bugs up when the renderer/nodebuilder gets them ready, but has no place to put them.