diff --git a/app/controllers/watchers_controller.rb b/app/controllers/watchers_controller.rb
index 5d5d2d50..2325634a 100644
--- a/app/controllers/watchers_controller.rb
+++ b/app/controllers/watchers_controller.rb
@@ -53,7 +53,7 @@ class WatchersController < ApplicationController
   end
 
   def destroy
-    @watched.set_watcher(User.find(params[:user_id]), false) if request.post?
+    @watched.set_watcher(Principal.find(params[:user_id]), false) if request.post?
     respond_to do |format|
       format.html { redirect_to :back }
       format.js do
diff --git a/test/functional/watchers_controller_test.rb b/test/functional/watchers_controller_test.rb
index 858e807e..ad839bac 100644
--- a/test/functional/watchers_controller_test.rb
+++ b/test/functional/watchers_controller_test.rb
@@ -132,6 +132,21 @@ class WatchersControllerTest < ActionController::TestCase
     assert Issue.find(2).watched_by?(User.find(7))
   end
 
+  context "POST :new" do
+    should "add groups" do
+      @group = Group.generate!.reload
+      Member.generate!(:project => Project.find(1), :roles => [Role.find(1)], :principal => @group)
+
+      @request.session[:user_id] = 2
+      assert_difference('Watcher.count') do
+        xhr :post, :new, :object_type => 'issue', :object_id => '2', :user_ids => [@group.id.to_s]
+        assert_response :success
+        assert_select_rjs :replace_html, 'watchers'
+      end
+      assert Issue.find(2).watched_by?(@group)
+    end
+  end
+
   def test_remove_watcher
     @request.session[:user_id] = 2
     assert_difference('Watcher.count', -1) do
@@ -141,4 +156,23 @@ class WatchersControllerTest < ActionController::TestCase
     end
     assert !Issue.find(2).watched_by?(User.find(3))
   end
+
+  context "POST :destroy" do
+    should "remove a group" do
+      @group = Group.generate!.reload
+      Member.generate!(:project => Project.find(1), :roles => [Role.find(1)], :principal => @group)
+      assert Issue.find(2).add_watcher(@group)
+      assert Issue.find(2).watched_by?(@group)
+
+      @request.session[:user_id] = 2
+      assert_difference('Watcher.count', -1) do
+        xhr :post, :destroy, :object_type => 'issue', :object_id => '2', :user_id => @group.id.to_s
+        assert_response :success
+        assert_select_rjs :replace_html, 'watchers'
+      end
+      assert !Issue.find(2).watched_by?(@group)
+    end
+    
+  end
+  
 end
diff --git a/vendor/plugins/acts_as_watchable/lib/acts_as_watchable.rb b/vendor/plugins/acts_as_watchable/lib/acts_as_watchable.rb
index 5fba3d76..9d99d2b7 100644
--- a/vendor/plugins/acts_as_watchable/lib/acts_as_watchable.rb
+++ b/vendor/plugins/acts_as_watchable/lib/acts_as_watchable.rb
@@ -42,7 +42,7 @@ module Redmine
         
         # Removes user from the watchers list
         def remove_watcher(user)
-          return nil unless user && user.is_a?(User)
+          return nil unless user && user.is_a?(Principal)
           Watcher.delete_all "watchable_type = '#{self.class}' AND watchable_id = #{self.id} AND user_id = #{user.id}"
         end