Problems with Railroad

Hello everybody i'm working with Railroad

and i want to create the diagram of all the model in my aplication
inlcuding the models of the plugins, bu when i run the command

railroad -p -o modelos_completo.dot -M

show me the next error:

C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/gems/
activesupport-2.3
.5/lib/active_support/whiny_nil.rb:52:in `method_missing': undefined
method `cla
ss_name' for nil:NilClass (NoMethodError)
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/acti
verecord-2.3.5/lib/active_record/reflection.rb:316:in
`derive_class_name'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/acti
verecord-2.3.5/lib/active_record/reflection.rb:112:in `class_name'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/rail
road-0.5.0/lib/railroad/models_diagram.rb:134:in `process_association'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/rail
road-0.5.0/lib/railroad/models_diagram.rb:102:in `process_class'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/rail
road-0.5.0/lib/railroad/models_diagram.rb:101:in `each'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/rail
road-0.5.0/lib/railroad/models_diagram.rb:101:in `process_class'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/rail
road-0.5.0/lib/railroad/models_diagram.rb:27:in `generate'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/rail
road-0.5.0/lib/railroad/models_diagram.rb:26:in `each'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/rail
road-0.5.0/lib/railroad/models_diagram.rb:26:in `generate'
        from C:/Program Files/BitNami RubyStack/ruby/lib/ruby/gems/1.8/
gems/rail
road-0.5.0/bin/railroad:47
        from C:/Program Files/BitNami RubyStack/ruby/bin/railroad:
19:in `load'
        from C:/Program Files/BitNami RubyStack/ruby/bin/railroad:19

and if i run railroad -a -o modelos_completo.dot -M

only include un model but where are the rest?

Can anyone help me? thanks