diff --git a/activerecord/lib/active_record/association_preload.rb b/activerecord/lib/active_record/association_preload.rb
index 313c51dae0..a89f5f5459 100644
--- a/activerecord/lib/active_record/association_preload.rb
+++ b/activerecord/lib/active_record/association_preload.rb
@@ -162,12 +162,13 @@ module ActiveRecord
 
           # Construct a mapping from klass to a list of ids to load and a mapping of those ids back to their parent_records
           records.each do |record|
-            klass = record.send(polymorph_type)
-            klass_id = record.send(primary_key_name)
+            if klass = record.send(polymorph_type)
+              klass_id = record.send(primary_key_name)
 
-            id_map = klasses_and_ids[klass] ||= {}
-            id_list_for_klass_id = (id_map[klass_id] ||= [])
-            id_list_for_klass_id << record
+              id_map = klasses_and_ids[klass] ||= {}
+              id_list_for_klass_id = (id_map[klass_id] ||= [])
+              id_list_for_klass_id << record
+            end
           end
           klasses_and_ids = klasses_and_ids.to_a
         else
diff --git a/activerecord/test/cases/associations/join_model_test.rb b/activerecord/test/cases/associations/join_model_test.rb
index e72c6d0e07..e128d6dda3 100644
--- a/activerecord/test/cases/associations/join_model_test.rb
+++ b/activerecord/test/cases/associations/join_model_test.rb
@@ -331,7 +331,7 @@ class AssociationsJoinModelTest < ActiveRecord::TestCase
     assert_no_queries do
       assert_equal desired, tag_with_include.tagged_posts
     end
-    assert_equal 4, tag_with_include.taggings.length
+    assert_equal 5, tag_with_include.taggings.length
   end
 
   def test_has_many_through_has_many_find_all
@@ -589,6 +589,12 @@ class AssociationsJoinModelTest < ActiveRecord::TestCase
     assert taggables.include?(posts(:welcome))
   end
 
+  def test_preload_nil_polymorphic_belongs_to
+    assert_nothing_raised do
+      taggings = Tagging.find(:all, :include => :taggable, :conditions => ['taggable_type IS NULL'])
+    end
+  end
+
   def test_preload_polymorphic_has_many
     posts               = Post.find(:all, :order => 'posts.id')
     posts_with_taggings = Post.find(:all, :include => :taggings, :order => 'posts.id')
diff --git a/activerecord/test/fixtures/taggings.yml b/activerecord/test/fixtures/taggings.yml
index 2213b15494..1e3d5965b8 100644
--- a/activerecord/test/fixtures/taggings.yml
+++ b/activerecord/test/fixtures/taggings.yml
@@ -22,4 +22,7 @@ godfather:
   tag_id: 1
   taggable_id: 1
   taggable_type: Item
-  
\ No newline at end of file
+
+orphaned:
+  id: 5
+  tag_id: 1