It doesn't work under a submodule directory

git_stats works fine for the root git directory. But it failed in submodule directory:

.rvm/gems/ruby-1.9.3-p484/gems/git_stats-1.0.13/lib/git_stats/generator.rb:21:in `validate_repo_path': . is not a git repository (ArgumentError)
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/gems/git_stats-1.0.13/lib/git_stats/generator.rb:8:in `initialize'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/gems/git_stats-1.0.13/lib/git_stats/cli.rb:18:in `new'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/gems/git_stats-1.0.13/lib/git_stats/cli.rb:18:in `generate'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/gems/git_stats-1.0.13/bin/git_stats:10:in `<top (required)>'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/bin/git_stats:23:in `load'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/bin/git_stats:23:in `<main>'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/bin/ruby_executable_hooks:15:in `eval'
	from /Users/4/.rvm/gems/ruby-1.9.3-p484/bin/ruby_executable_hooks:15:in `<main>'