[BUG?] "format-patch --range-diff=$commit HEAD^ HEAD" fails

Immediately after "git commit --amend" of a single commit, this fails:

    $ git format-patch --range-diff=@{1} -v2 HEAD^ HEAD
    fatal: --range-diff requires --cover-letter or single patch

The intent is very clear. I know it is a single-patch topic, and I want to show what I changed with "commit --amend" as a reference to the previous version. These, on the other hand, that specify the range of commits to be emitted correctly recognises that we are talking about a single patch and does not fail.

    $ git format-patch --range-diff=@{1} -v2 HEAD^
    $ git format-patch --range-diff=@{1} -v2 -1

It seems to me that the traditional "format-patch <since> <until>" notation, which is understood by everywhere else in format-patch, is not correctly understood by the code for "--range-diff" to make this complaint, to recognise that the given range has only a single commit.