$> thor example:zoom --foo convert.thor
An exception occurred running /home/jesse/.rvm/gems/rbx-head/bin/thor
    Coercion error: nil.to_str => String failed (TypeError)

Backtrace:
                        Type.coerce_to at kernel/common/type.rb:22
             Kernel(Class)#StringValue at kernel/common/kernel.rb:112
                 File.exists? (exist?) at kernel/common/file.rb:344
           Thor::Sandbox::Example#zoom at example.thor:9
                       Thor::Task#run at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor/task.rb:22
 Thor::Invocation(Thor::Sandbox::Example)#invoke_task at /home/jesse/.rvm/gems
                                                         /rbx-head/gems
                                                         /thor-0.14.6/lib/thor
                                                         /invocation.rb:118
                        Thor.dispatch at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor.rb:263
 Thor::Base::ClassMethods(Class)#start at /home/jesse/.rvm/gems/rbx-head/gems
                                          /thor-0.14.6/lib/thor/base.rb:389
 Thor::Runner#example:zoom (method_missing) at /home/jesse/.rvm/gems/rbx-head
                                               /gems/thor-0.14.6/lib/thor
                                               /runner.rb:34
    Thor::Task(Thor::DynamicTask)#run at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor/task.rb:22
                Thor::DynamicTask#run at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor/task.rb:108
 Thor::Invocation(Thor::Runner)#invoke_task at /home/jesse/.rvm/gems/rbx-head
                                               /gems/thor-0.14.6/lib/thor
                                               /invocation.rb:118
                        Thor.dispatch at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor.rb:263
 Thor::Base::ClassMethods(Class)#start at /home/jesse/.rvm/gems/rbx-head/gems
                                          /thor-0.14.6/lib/thor/base.rb:389
                      main.__script__ at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/bin/thor:6
                   Kernel(Object)#load at kernel/common/kernel.rb:684
                      main.__script__ at /home/jesse/.rvm/gems/rbx-head/bin
                                         /thor:19
      Rubinius::CodeLoader#load_script at kernel/delta/codeloader.rb:67
      Rubinius::CodeLoader.load_script at kernel/delta/codeloader.rb:91
               Rubinius::Loader#script at kernel/loader.rb:518
                 Rubinius::Loader#main at kernel/loader.rb:616
                 Rubinius::Loader.main at kernel/loader.rb:655
                     Object#__script__ at kernel/loader.rb:666

Caused by: undefined method `to_str' on nil:NilClass. (NoMethodError)

Backtrace:
  Kernel(NilClass)#to_str (method_missing) at kernel/delta/kernel.rb:85
                        Type.coerce_to at kernel/common/type.rb:20
             Kernel(Class)#StringValue at kernel/common/kernel.rb:112
                 File.exists? (exist?) at kernel/common/file.rb:344
           Thor::Sandbox::Example#zoom at example.thor:9
                       Thor::Task#run at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor/task.rb:22
 Thor::Invocation(Thor::Sandbox::Example)#invoke_task at /home/jesse/.rvm/gems
                                                         /rbx-head/gems
                                                         /thor-0.14.6/lib/thor
                                                         /invocation.rb:118
                        Thor.dispatch at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor.rb:263
 Thor::Base::ClassMethods(Class)#start at /home/jesse/.rvm/gems/rbx-head/gems
                                          /thor-0.14.6/lib/thor/base.rb:389
 Thor::Runner#example:zoom (method_missing) at /home/jesse/.rvm/gems/rbx-head
                                               /gems/thor-0.14.6/lib/thor
                                               /runner.rb:34
    Thor::Task(Thor::DynamicTask)#run at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor/task.rb:22
                Thor::DynamicTask#run at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor/task.rb:108
 Thor::Invocation(Thor::Runner)#invoke_task at /home/jesse/.rvm/gems/rbx-head
                                               /gems/thor-0.14.6/lib/thor
                                               /invocation.rb:118
                        Thor.dispatch at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/lib/thor.rb:263
 Thor::Base::ClassMethods(Class)#start at /home/jesse/.rvm/gems/rbx-head/gems
                                          /thor-0.14.6/lib/thor/base.rb:389
                      main.__script__ at /home/jesse/.rvm/gems/rbx-head/gems
                                         /thor-0.14.6/bin/thor:6
                   Kernel(Object)#load at kernel/common/kernel.rb:684
                      main.__script__ at /home/jesse/.rvm/gems/rbx-head/bin
                                         /thor:19
      Rubinius::CodeLoader#load_script at kernel/delta/codeloader.rb:67
      Rubinius::CodeLoader.load_script at kernel/delta/codeloader.rb:91
               Rubinius::Loader#script at kernel/loader.rb:518
                 Rubinius::Loader#main at kernel/loader.rb:616
                 Rubinius::Loader.main at kernel/loader.rb:655
                     Object#__script__ at kernel/loader.rb:666