`git fetch origin ''` (the empty string) -- should this work?

this seems to end up doing the same as `git fetch origin HEAD` but I
can't figure out why -- perhaps someone on this list can help me :)

$ git fetch origin ''
From github.com:python/cpython
 * branch                  HEAD       -> FETCH_HEAD

what I expected was `fatal: Couldn't find remote ref ''` or something
of the sort as would be seen when fetching some non-existent branch:

$ git fetch origin wat
fatal: Couldn't find remote ref wat

Anthony