I have problem with acts_as_inviteable plugin for rails app, how can I get rid of this problem??

Question

The following error is occurred when I call @invitation = Invitation.new()....

#<ArgumentError: Unknown key(s): primary_key>
Unknown key(s): primary_key
C:/Ruby/lib/ruby/gems/1.8/gems/activesupport-2.3.2/lib/active_support/core_ext/hash/keys.rb:47:in `assert_valid_keys'
C:/Ruby/lib/ruby/gems/1.8/gems/activerecord-2.3.2/lib/active_record/associations.rb:1506:in `create_belongs_to_reflection'
C:/Ruby/lib/ruby/gems/1.8/gems/activerecord-2.3.2/lib/active_record/associations.rb:994:in `belongs_to'
(eval):3:in `belongs_to'
D:/Amit/Development/TCH/vendor/plugins/acts_as_inviteable/lib/acts_as/invitation.rb:21:in `acts_as_invitation'
D:/Amit/Development/TCH/app/models/invitation.rb:2

Can anybody help me??