如何让Rye在ruby中返回无效命令

以下是我正在使用的代码：

require 'rye'
r = Rye::Box.new("ip_of_server", :user => "username_of_server", :password => "password_of_server", :safe => false)
r.execute("which testing")

我得到以下信息：

(cmd: which testing; status: 1)
    from /var/lib/gems/2.3.0/gems/rye-0.9.13/lib/rye/box.rb:855:in `rescue in run_command'
    from /var/lib/gems/2.3.0/gems/rye-0.9.13/lib/rye/box.rb:804:in `run_command'
    from /var/lib/gems/2.3.0/gems/rye-0.9.13/lib/rye/box.rb:473:in `method_missing'
    from (irb):64
    from /usr/bin/irb:11:in `<main>'

    irb(main):012:0* begin
    irb(main):013:1*   r.execute("which testing")
    irb(main):014:1> rescue => e
    irb(main):015:1>   merp = e.message
    irb(main):016:1> end