bug report? fatal: Ambiguous object name: 'xxx' during branch operation

If the default start-point for a branch is HEAD, then how can it fail
due to ambiguity?  Or what the heck is going on here? :-)

$ git branch -a
* 1.1-SNAPSHOT
  master
  remotes/1.1-SNAPSHOT
  remotes/trunk
$

$ git branch getProviders
warning: refname '1.1-SNAPSHOT' is ambiguous.
fatal: Ambiguous object name: '1.1-SNAPSHOT'.
$

$ cat .git/HEAD
ref: refs/heads/1.1-SNAPSHOT
$

$ git branch getProviders refs/heads/1.1-SNAPSHOT
$
success!


[svn-remote "svn"]
url = https://host/svn/all
fetch = GHIE/maven-projects/hie-util/trunk:refs/remotes/trunk
branches = GHIE/maven-projects/hie-util/branches/*:refs/remotes/*
tags = GHIE/maven-projects/hie-util/tags/*:refs/remotes/tags/*

Thanks,
Arya

P.S.  I'll use --prefix for svn to avoid this in the future!