Index: actionpack/test/template/date_helper_test.rb
===================================================================
--- actionpack/test/template/date_helper_test.rb	(revision 8440)
+++ actionpack/test/template/date_helper_test.rb	(working copy)
@@ -17,6 +17,13 @@
         '123'
       end
     end
+    
+    NewPost = Struct.new("Post", :id, :written_on, :updated_at)
+    NewPost.class_eval do
+      def to_param
+        ''
+      end
+    end
   end
 
   def test_distance_in_words
@@ -885,7 +892,26 @@
 
     assert_equal expected, date_select("post[]", "written_on")
   end
+  
+  def test_date_select_with_auto_index_when_id_is_nil
+    @post = NewPost.new
+    @post.written_on = Date.new(2004, 6, 15)
 
+    expected = %{<select id="post__written_on_1i" name="post[][written_on(1i)]">\n}
+    expected << %{<option value="1999">1999</option>\n<option value="2000">2000</option>\n<option value="2001">2001</option>\n<option value="2002">2002</option>\n<option value="2003">2003</option>\n<option value="2004" selected="selected">2004</option>\n<option value="2005">2005</option>\n<option value="2006">2006</option>\n<option value="2007">2007</option>\n<option value="2008">2008</option>\n<option value="2009">2009</option>\n}
+    expected << "</select>\n"
+
+    expected << %{<select id="post__written_on_2i" name="post[][written_on(2i)]">\n}
+    expected << %{<option value="1">January</option>\n<option value="2">February</option>\n<option value="3">March</option>\n<option value="4">April</option>\n<option value="5">May</option>\n<option value="6" selected="selected">June</option>\n<option value="7">July</option>\n<option value="8">August</option>\n<option value="9">September</option>\n<option value="10">October</option>\n<option value="11">November</option>\n<option value="12">December</option>\n}
+    expected << "</select>\n"
+
+    expected << %{<select id="post__written_on_3i" name="post[][written_on(3i)]">\n}
+    expected << %{<option value="1">1</option>\n<option value="2">2</option>\n<option value="3">3</option>\n<option value="4">4</option>\n<option value="5">5</option>\n<option value="6">6</option>\n<option value="7">7</option>\n<option value="8">8</option>\n<option value="9">9</option>\n<option value="10">10</option>\n<option value="11">11</option>\n<option value="12">12</option>\n<option value="13">13</option>\n<option value="14">14</option>\n<option value="15" selected="selected">15</option>\n<option value="16">16</option>\n<option value="17">17</option>\n<option value="18">18</option>\n<option value="19">19</option>\n<option value="20">20</option>\n<option value="21">21</option>\n<option value="22">22</option>\n<option value="23">23</option>\n<option value="24">24</option>\n<option value="25">25</option>\n<option value="26">26</option>\n<option value="27">27</option>\n<option value="28">28</option>\n<option value="29">29</option>\n<option value="30">30</option>\n<option value="31">31</option>\n}
+    expected << "</select>\n"
+
+    assert_equal expected, date_select("post[]", "written_on")
+  end
+
   def test_date_select_with_different_order
     @post = Post.new
     @post.written_on = Date.new(2004, 6, 15)
Index: actionpack/lib/action_view/helpers/date_helper.rb
===================================================================
--- actionpack/lib/action_view/helpers/date_helper.rb	(revision 8440)
+++ actionpack/lib/action_view/helpers/date_helper.rb	(working copy)
@@ -645,7 +645,7 @@
           prefix = "#{@object_name}"
           if options[:index]
             prefix << "[#{options[:index]}]"
-          elsif @auto_index
+          elsif defined?(@auto_index)
             prefix << "[#{@auto_index}]"
           end
           options.merge(:prefix => "#{prefix}[#{@method_name}(#{position}i)]")