undefined method 'each' in acts_as_taggable_on with rails 3 beta 3 and ruby 1.9

Hi

I am trying to use acts_as_taggable_on in my new projects. But I got
Error when I try to save tags.
undefined method `each' for "[]":String

NoMethodError: undefined method `each' for "":String
        from C:/Ruby19/lib/ruby/gems/1.9.1/gems/
activerecord-3.0.0.beta3/lib/active_record/associations/
association_collection.rb:347:in `replace'
        from C:/Ruby19/lib/ruby/gems/1.9.1/gems/
activerecord-3.0.0.beta3/lib/active_record/associations.rb:1415:in
`block in collection_accessor_methods'
        from (irb):6
        from C:/Ruby19/lib/ruby/gems/1.9.1/gems/railties-3.0.0.beta3/
lib/rails/commands/console.rb:47:in `start'
        from C:/Ruby19/lib/ruby/gems/1.9.1/gems/railties-3.0.0.beta3/
lib/rails/commands/console.rb:8:in `start'
        from C:/Ruby19/lib/ruby/gems/1.9.1/gems/railties-3.0.0.beta3/
lib/rails/commands.rb:34:in `<top (required)>'
        from script/rails:9:in `require'
        from script/rails:9:in `<main>'

I guess it maybe because ruby 1.9 doesn't support each for string.

Does anyone have the same problem? Any solution?

Cheers,
Danyi