(At first sight) unexpected git-diff behavior

Hello,

I often specify revision ranges when I need a diff, because I find it
easier to see what is diffed against what:

git diff v1.5.0-rc0..v1.5.0-rc1

Today I tried to do the same to get a diff between linux v2.6.11 (which
is a tag for a tree object) and v2.6.12:

$ git diff v2.6.11..v2.6.12
error: Object 5dc01c595e6c6ec9ccda4f6f69c131c0dd945f8c is a tree, not a
commit
fatal: Invalid revision range v2.6.11..v2.6.12

Of course, git diff v2.6.11 v2.6.12 works as it should.

I am not sure if git diff should be able to work with tree objects in
revision ranges (which does not really make sense).

Thoughts?