Strange behaviour when returning an array from class_eval'ed method

With Ruby 1.9.2, I'm using class_eval to extend a class.

def slugged(fields)
  # assign string to variable only for easier debugging
  method = <<-EOS
    def slug_fields
      #{ fields.is_a?(Array) ? fields.inspect : ":#{ fields }" }
    end
  EOS

  class_eval method
end

This works fine as long as fields is a symbol (e.g. after slugged :name, slug_fields returns :name).

However, calling slugged with an array makes slug_fields returns nil (e.g. after slugged [:kicker, :headline], slug_fields returns nil).

Strangely, when debugging slugged, the string containing the to-be-created method looks exactly the way you would expect them to:

"          def slug_fields\n            [:kicker, :headline]\n          end\n"
"          def slug_fields\n            :name\n          end\n"开发者_如何学Python

edit: as requested, a more complete version of what breaks for me:

module Extensions
  module Slugged
    extend ActiveSupport::Concern

    included do
      before_validation { |record| record.slug ||= record.sluggerize }
    end 

    module ClassMethods 

      def slugged(fields)
        # assign string to variable only for easier debugging
        method = <<-EOS
          def slug_fields
            #{ fields.is_a?(Array) ? fields.inspect : ":#{ fields }" }
          end
        EOS

        class_eval method
      end
    end

    module InstanceMethods
      def sluggerize
        fields = slug_fields

        slug_string = case
          when fields.is_a?(Array)
            fields.map { |f| self.send(f) }.join('-')
          else
            self.send fields
          end

        slug_string.parameterize
      end
    end
  end
end

class Article < ActiveRecord::Base
  include Extensions::Slugged  
  slugged [:kicker, :headline]
end

class Station < ActiveRecord::Base
  include Extensions::Slugged
  slugged :name
end

a = Article.new :headline => "this is a great headline!", :kicker => "attention-drawing kicker"
a.save # works, slug is set

s = Station.new :name => "Great Music"
s.save # TypeError: nil is not a symbol (in sluggerize where "self.send fields" is called)

Your code works fine for me under 1.9.2:

class Foo
  class << self
    def slugged(fields)
      method = <<-EOS
        def slug_fields
          #{ fields.is_a?(Array) ? fields.inspect : ":#{ fields }" }
        end
      EOS
      class_eval method
    end
  end
end

Foo.slugged :a
p Foo.new.slug_fields
#=> :a

Foo.slugged [:a,:b]
p Foo.new.slug_fields
#=> [:a, :b]

p RUBY_DESCRIPTION
#=> "ruby 1.9.2p180 (2011-02-18) [i386-mingw32]"

Can you please provide a complete, runnable, standalone test case that breaks for you?