ruby-debug failure in rails2.1

when i use debugger in the spec , after type irb , it raise error below

INTERNAL ERROR!!! wrong number of arguments (2 for 1)
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb/extend-command.rb:157:in
`respond_to?'
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb/extend-command.rb:157:in
`install_alias_method'
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb/extend-command.rb:179:in
`extend_object'
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb/extend-command.rb:178:in
`each'
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb/extend-command.rb:178:in
`extend_object'
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb.rb:93:in
`extend'
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb.rb:93:in
`initialize'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/commands/irb.rb:24:in
`new'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/commands/irb.rb:24:in
`start_session'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/commands/irb.rb:62:in
`execute'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/processor.rb:272:in
`one_cmd'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/processor.rb:259:in
`process_commands'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/processor.rb:258:in
`each'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/processor.rb:258:in
`process_commands'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/processor.rb:251:in
`catch'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/processor.rb:251:in
`process_commands'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-0.10.1/cli/ruby-debug/processor.rb:171:in
`__at_line'
  (eval):5:in `at_line'
  (eval):3:in `synchronize'
  (eval):3:in `at_line'
  /Library/Ruby/Gems/1.8/gems/ruby-debug-base-0.10.1/lib/ruby-debug-base.rb:52:in
`at_line'
  ./spec/models/diagnosis_spec.rb:16
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/example/example_methods.rb:84:in
`instance_eval'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/example/example_methods.rb:84:in
`run_with_description_capturing'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/example/example_methods.rb:21:in
`execute'
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/timeout.rb:48:in
`timeout'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/example/example_methods.rb:18:in
`execute'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/example/example_group_methods.rb:303:in
`execute_examples'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/example/example_group_methods.rb:302:in
`each'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/example/example_group_methods.rb:302:in
`execute_examples'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/example/example_group_methods.rb:130:in
`run'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/runner/example_group_runner.rb:22:in
`run'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/runner/example_group_runner.rb:21:in
`each'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/runner/example_group_runner.rb:21:in
`run'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/runner/options.rb:106:in
`run_examples'
  /Users/sunsware/sandbox/dxj/vendor/plugins/rspec/lib/spec/runner/command_line.rb:19:in
`run'
  ./script/spec:4...