rspec - why does be_valid not work

I'm trying to do this:

  describe "should fail on create if the name is shorter than 5 characters" do
    group = Factory.build(:group, :name => "a")
    group.should be_invalid
    group.should have(1).error_on(:name)
  end

But I get the following error:

/spec/models/group_model_spec.rb:22: undefined local variable or method `be_valid' for #<Class:0x105d13ed0> (NameError)
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/example_group.rb:132:in `module_eval'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/example_group.rb:132:in `subclass'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/example_group.rb:119:in `describe'
    from /xxxx/spec/models/group_model_spec.rb:15
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/example_group.rb:132:in `module_eval'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/example_group.rb:132:in `subclass'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/example_group.rb:119:in `describe'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/extensions/object.rb:6:in `describe'
    from /Users/bhel开发者_Python百科lman/Sites/companyline/spec/models/group_model_spec.rb:3
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/configuration.rb:386:in `load'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/configuration.rb:386:in `load_spec_files'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/configuration.rb:386:in `map'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/configuration.rb:386:in `load_spec_files'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/command_line.rb:18:in `run'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/runner.rb:55:in `run_in_process'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/runner.rb:46:in `run'
    from /Library/Ruby/Gems/1.8/gems/rspec-core-2.5.1/lib/rspec/core/runner.rb:10:in `autorun'
    from /usr/bin/rspec:19

Ideas? Thanks

This is the correct syntax :

group.should_not be_valid