branch --set-upstream-to unexpectedly fails with "starting point ... is no branch"

There is a strange "branch --set-upstream-to" failure for "clones" which haven't been created using "git clone" but constructed using "git init", "git remote add" and "git fetch".

Following script first creates a "main" repository and then constructs the clone. Finally, in the clone branches origin/1 and origin/2 will be present, however it's not possible to invoke "git branch --set-upstream-to" for origin/2 (it works fine for origin/1).

I guess the behavior is related to following line in .git/config:

fetch = refs/heads/1:refs/remotes/origin/1

However, I don't understand what's the problem for Git here? Definitely the error "starting point 'origin/2' is not a branch" is wrong.

$ git --version git version 2.5.0.windows.1

$ cd /tmp/gittest
$ mkdir main
$ cd main
$ git init
$ touch file
$ git add file
$ git commit -m "import"
$ git branch 1
$ git branch 2
$ git branch
   1
   2
* master

$ cd /tmp/gittest
$ mkdir clone
$ cd clone
$ git init
Initialized empty Git repository in /tmp/gittest/clone/.git/
$ git remote add origin /tmp/gittest/main
$ git config remote.origin.fetch refs/heads/1:refs/remotes/origin/1
$ git fetch origin
remote: Counting objects: 3, done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
 From /tmp/gittest/main
  * [new branch]      1          -> origin/1

$ git fetch origin refs/heads/2:refs/remotes/origin/2
 From /tmp/gittest/main
  * [new branch]      2          -> origin/2

$ git branch --no-track 2 refs/remotes/origin/2
$ git branch
   2

# HERE COMES THE STRANGE FAILURE: $ git branch --set-upstream-to=origin/2 2 fatal: Cannot setup tracking information; starting point 'origin/2' is not a branch.

# THIS WORKS AS EXPECTED: $ git branch --set-upstream-to=origin/1 2 Branch 2 set up to track remote branch 1 from origin by rebasing.

-Marc