diff --git a/app/models/rate.rb b/app/models/rate.rb
index e1fe998..a8e766e 100644
--- a/app/models/rate.rb
+++ b/app/models/rate.rb
@@ -1,4 +1,6 @@
 class Rate < ActiveRecord::Base
+  class InvalidParameterException < Exception; end
+
   belongs_to :project
   belongs_to :user
   has_many :time_entries
@@ -27,9 +29,9 @@ 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
-    return nil unless user.is_a?(User)
-    return nil unless project.nil? || project.is_a?(Project)
-    
+    raise Rate::InvalidParameterException.new("user must be a User instance") unless user.is_a?(User)
+    raise Rate::InvalidParameterException.new("project must be a Project instance") unless project.nil? || project.is_a?(Project)
+
     rate = self.for_user_project_and_date(user, project, date)
     
     return nil if rate.nil?
diff --git a/spec/models/rate_spec.rb b/spec/models/rate_spec.rb
index 5868e57..de22a27 100644
--- a/spec/models/rate_spec.rb
+++ b/spec/models/rate_spec.rb
@@ -191,16 +191,20 @@ describe Rate, 'for' do
     end
   end
   
-  it 'with an invalid user should be nil' do
+  it 'with an invalid user should raise an InvalidParameterException' do
     object = mock('random_object_with_id_attribute')
     Rate.should_not_receive(:for_user_project_and_date)
-    Rate.for(object).should be_nil
+    lambda {
+      Rate.for(object)
+    }.should raise_error(Rate::InvalidParameterException, "user must be a User instance")
   end
   
   it 'with an invalid project should be nil' do
     object = mock('random_object_with_id_attribute')
     Rate.should_not_receive(:for_user_project_and_date)
-    Rate.for(@user, object).should be_nil
+    lambda {
+      Rate.for(@user, object)
+    }.should raise_error(Rate::InvalidParameterException, "project must be a Project instance")
   end
   
 end