pre-commit: Could not find valid gem __fake_gem

I’m having trouble getting the hooks installed on OSX:

[INFO] Installing environment for https://github.com/chriskuehl/puppet-pre-commit-hooks.git.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: Command: (u'/bin/bash', u'/Users/ray.ward/.rbenv/shims/gem', 'install', '--no-ri', '--no-rdoc', '__fake_gem-0.gem')
Return code: 2
Expected return code: 0
Output: (none)
Errors:
    ERROR:  Could not find a valid gem '__fake_gem-0.gem' (>= 0) in any repository
    ERROR:  Possible alternatives: fake_gem, fake-gem, fake_gem_push


Check the log at ~/.pre-commit/pre-commit.log
$ ruby -v
ruby 2.1.6p336 (2015-04-13 revision 50298) [x86_64-darwin14.0]
$ gem -v
2.2.3
$ pre-commit -V
pre-commit 0.9.1
$ uname -a
Darwin SYD-MBP-WARD 15.6.0 Darwin Kernel Version 15.6.0: Mon Aug 29 20:21:34 PDT 2016; root:xnu-3248.60.11~1/RELEASE_X86_64 x86_64

any ideas?