diff --git a/app/models/rate.rb b/app/models/rate.rb
index 9343e65..2540200 100644
--- a/app/models/rate.rb
+++ b/app/models/rate.rb
@@ -40,7 +40,11 @@ class Rate < ActiveRecord::Base
   # API to find the Rate for a +user+ on a +project+ at a +date+
   def self.for(user, project = nil, date = Date.today.to_s)
     # Check input since it's a "public" API
-    raise Rate::InvalidParameterException.new("user must be a User instance") unless user.is_a?(User)
+    if Object.const_defined? 'Group' # 0.8.x compatibility
+      raise Rate::InvalidParameterException.new("user must be a Principal instance") unless user.is_a?(Principal)
+    else
+      raise Rate::InvalidParameterException.new("user must be a User instance") unless user.is_a?(User)
+    end
     raise Rate::InvalidParameterException.new("project must be a Project instance") unless project.nil? || project.is_a?(Project)
     Rate.check_date_string(date)
       
diff --git a/lib/rate_project_hook.rb b/lib/rate_project_hook.rb
index 0cf098a..0819d1f 100644
--- a/lib/rate_project_hook.rb
+++ b/lib/rate_project_hook.rb
@@ -28,7 +28,13 @@ class RateProjectHook < Redmine::Hook::ViewListener
 
     return '' unless (User.current.allowed_to?(:view_rate, project) || User.current.admin?)
 
-    rate = Rate.for(member.user, project)
+    if Object.const_defined? 'Group' # 0.8.x compatibility
+      # Groups cannot have a rate
+      return content_tag(:td,'') if member.principal.is_a? Group
+      rate = Rate.for(member.principal, project)
+    else
+      rate = Rate.for(member.user, project)
+    end
 
     content = ''
     
diff --git a/spec/models/rate_spec.rb b/spec/models/rate_spec.rb
index 570b050..d068ca9 100644
--- a/spec/models/rate_spec.rb
+++ b/spec/models/rate_spec.rb
@@ -222,7 +222,7 @@ describe Rate, 'for' do
     Rate.should_not_receive(:for_user_project_and_date)
     lambda {
       Rate.for(object)
-    }.should raise_error(Rate::InvalidParameterException, "user must be a User instance")
+    }.should raise_error(Rate::InvalidParameterException, "user must be a Principal instance")
   end
   
   it 'with an invalid project should raise an InvalidParameterException' do