Undefined method `stringify_keys!' for

plz tell me why the problem is coming…

def test_should_update_filter_criterion
@request.session[:current_context_id] =
user_context_roles(:jsmith_admin_medidata).context_id
@request.session[:current_user_id] =
user_context_roles(:jsmith_admin_medidata).id
put :update_filter_criterion, {:id=>filters(:filter1).id,
:filter_criterion=>filter_criteria(:filter_criterion1) }
assert_response :success
end

test_should_update_filter_criterion(FiltersControllerTest):
NoMethodError: undefined method stringify_keys!' for #<FilterCriterion:0x48878c8> C:/ruby/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/base.rb:1860:inmethod_missing’
C:/ruby/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/base.rb:1668:in
attributes=' C:/ruby/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/base.rb:1591:inupdate_attributes’