I tried to suggest an edit to the tags on this question, by removing the one tag it had, and adding two others I felt were more appropriate. I didn't find anything to change in the content.
However, when I clicked "Save Edits" I got the following message:
Edits must be at least 6 characters; is there something else to improve in this post?
and the edit was not submitted. I tried starting over from scratch and got the same result.
I believe that on other sites I've been allowed to do edits that simply changed the tags. This meta question indicates that it's a normal thing to do. Isn't it supposed to be the same, here?
I was able to get around it by making changes to the markup that don't change the way the content appears. Why wouldn't it work otherwise?