leap_cli: cannot load such file -- debugger

because i need both the master and develop version on my machine, i call the leap binaries from their source dir as described in the README. that works fine for almost things, but i cannot use the "--debug" flag:

» /home/varac/leap/git/leap_cli_master/bin/leap --version
leap 1.5.7, ruby 1.9.3

» /home/varac/leap/git/leap_cli_master/bin/leap --debug deploy single 
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- debugger (LoadError)
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /home/varac/leap/git/leap_cli_master/bin/leap:4:in `'

(from redmine: created on 2014-08-25, closed on 2014-12-10)