diff --git a/actionpack/lib/action_controller/session/cookie_store.rb b/actionpack/lib/action_controller/session/cookie_store.rb
index 5a728d1..174d3e1 100644
--- a/actionpack/lib/action_controller/session/cookie_store.rb
+++ b/actionpack/lib/action_controller/session/cookie_store.rb
@@ -110,7 +110,7 @@ module ActionController
           cookie = build_cookie(@key, cookie.merge(options))
           case headers[HTTP_SET_COOKIE]
           when Array
-            headers[HTTP_SET_COOKIE] << cookie
+            headers[HTTP_SET_COOKIE] = cookie
           when String
             headers[HTTP_SET_COOKIE] = [headers[HTTP_SET_COOKIE], cookie]
           when nil
diff --git a/actionpack/test/controller/session/cookie_store_test.rb b/actionpack/test/controller/session/cookie_store_test.rb
index 95d2eb1..da7ad9f 100644
--- a/actionpack/test/controller/session/cookie_store_test.rb
+++ b/actionpack/test/controller/session/cookie_store_test.rb
@@ -92,8 +92,8 @@ class CookieStoreTest < ActionController::IntegrationTest
     with_test_route_set do
       get '/set_session_value'
       assert_response :success
-      assert_equal ["_myapp_session=#{response.body}; path=/; httponly"],
-        headers['Set-Cookie']
+      assert_equal "_myapp_session=#{response.body}; path=/; httponly",
+                   headers['Set-Cookie']
    end
   end
 
@@ -141,13 +141,41 @@ class CookieStoreTest < ActionController::IntegrationTest
     end
   end
 
+  def test_uses_correct_session_cookie_after_reset_session
+    with_test_route_set do
+      old_session_key = "BAh7BjoIZm9vIghiYXI%3D--" +
+        "fef868465920f415f2c0652d6910d3af288a0367"
+
+      cookies[SessionKey] = old_session_key
+
+      get '/call_reset_session'
+      
+      assert_not_equal cookies[SessionKey], old_session_key
+    end
+  end
+
+  def test_sets_correct_set_cookie_header_after_reset_session
+    with_test_route_set do
+      old_session_key = "BAh7BjoIZm9vIghiYXI%3D--" +
+        "fef868465920f415f2c0652d6910d3af288a0367"
+
+      cookies[SessionKey] = old_session_key
+
+      get '/call_reset_session'
+      
+      assert cookies[SessionKey], old_session_key
+      assert_equal "_myapp_session=#{cookies[SessionKey]}; path=/; httponly",
+                   headers['Set-Cookie']
+    end
+  end
+
   def test_setting_session_value_after_session_reset
     with_test_route_set do
       get '/set_session_value'
       assert_response :success
       session_payload = response.body
-      assert_equal ["_myapp_session=#{response.body}; path=/; httponly"],
-        headers['Set-Cookie']
+      assert_equal "_myapp_session=#{response.body}; path=/; httponly",
+                    headers['Set-Cookie']
 
       get '/call_reset_session'
       assert_response :success
@@ -191,7 +219,7 @@ class CookieStoreTest < ActionController::IntegrationTest
       assert_response :success
 
       cookie_body = response.body
-      assert_equal ["_myapp_session=#{cookie_body}; path=/; expires=#{expected_expiry}; httponly"], headers['Set-Cookie']
+      assert_equal "_myapp_session=#{cookie_body}; path=/; expires=#{expected_expiry}; httponly", headers['Set-Cookie']
 
       # Second request does not access the session
       time = Time.local(2008, 4, 25)
@@ -201,7 +229,8 @@ class CookieStoreTest < ActionController::IntegrationTest
       get '/no_session_access'
       assert_response :success
 
-      assert_equal ["_myapp_session=#{cookie_body}; path=/; expires=#{expected_expiry}; httponly"], headers['Set-Cookie']
+      assert_equal "_myapp_session=#{cookie_body}; path=/; expires=#{expected_expiry}; httponly", 
+                   headers['Set-Cookie']
     end
   end