diff --git a/protos/google/protobuf/unittest_optimize_for.proto b/protos/google/protobuf/unittest_optimize_for.proto
index 390b37b23b..8e4ec11324 100644
--- a/protos/google/protobuf/unittest_optimize_for.proto
+++ b/protos/google/protobuf/unittest_optimize_for.proto
@@ -55,6 +55,7 @@ message TestOptimizedForSize {
 
   extend TestOptimizedForSize {
     optional int32 test_extension = 1234;
+    optional TestRequiredOptimizedForSize test_extension2 = 1235;
   }
 }
 
diff --git a/src/ProtocolBuffers.Test/GeneratedMessageTest.cs b/src/ProtocolBuffers.Test/GeneratedMessageTest.cs
index feba6e3b14..f0be393c54 100644
--- a/src/ProtocolBuffers.Test/GeneratedMessageTest.cs
+++ b/src/ProtocolBuffers.Test/GeneratedMessageTest.cs
@@ -287,5 +287,29 @@ namespace Google.ProtocolBuffers {
           ).BuildPartial();
       Assert.IsTrue(message.IsInitialized);
     }
+
+    [Test]
+    public void TestOptimizedForSizeMergeUsesAllFieldsFromTarget() {
+      TestOptimizedForSize withFieldSet = new TestOptimizedForSize.Builder { I = 10 }.Build();
+      TestOptimizedForSize.Builder builder = new TestOptimizedForSize.Builder();
+      builder.MergeFrom(withFieldSet);
+      TestOptimizedForSize built = builder.Build();
+      Assert.AreEqual(10, built.I);
+    }
+
+    [Test]
+    public void UninitializedExtensionInOptimizedForSizeMakesMessageUninitialized() {
+      TestOptimizedForSize.Builder builder = new TestOptimizedForSize.Builder();
+      builder.SetExtension(TestOptimizedForSize.TestExtension2,
+          new TestRequiredOptimizedForSize.Builder().BuildPartial());
+      Assert.IsFalse(builder.IsInitialized);
+      Assert.IsFalse(builder.BuildPartial().IsInitialized);
+
+      builder = new TestOptimizedForSize.Builder();
+      builder.SetExtension(TestOptimizedForSize.TestExtension2,
+          new TestRequiredOptimizedForSize.Builder { X = 10 }.BuildPartial());
+      Assert.IsTrue(builder.IsInitialized);
+      Assert.IsTrue(builder.BuildPartial().IsInitialized);
+    }
   }
 }
diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
index 135e7a2fe9..cc555777fe 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
@@ -17,15 +17,18 @@ namespace Google.ProtocolBuffers.TestProtos {
         "Citnb29nbGUvcHJvdG9idWYvdW5pdHRlc3Rfb3B0aW1pemVfZm9yLnByb3Rv" + 
         "EhFwcm90b2J1Zl91bml0dGVzdBokZ29vZ2xlL3Byb3RvYnVmL2NzaGFycF9v" + 
         "cHRpb25zLnByb3RvGiBnb29nbGUvcHJvdG9idWYvZGVzY3JpcHRvci5wcm90" + 
-        "bxoeZ29vZ2xlL3Byb3RvYnVmL3VuaXR0ZXN0LnByb3RvIp4BChRUZXN0T3B0" + 
+        "bxoeZ29vZ2xlL3Byb3RvYnVmL3VuaXR0ZXN0LnByb3RvIpICChRUZXN0T3B0" + 
         "aW1pemVkRm9yU2l6ZRIJCgFpGAEgASgFEi4KA21zZxgTIAEoCzIhLnByb3Rv" + 
         "YnVmX3VuaXR0ZXN0LkZvcmVpZ25NZXNzYWdlKgkI6AcQgICAgAIyQAoOdGVz" + 
         "dF9leHRlbnNpb24SJy5wcm90b2J1Zl91bml0dGVzdC5UZXN0T3B0aW1pemVk" + 
-        "Rm9yU2l6ZRjSCSABKAUiKQocVGVzdFJlcXVpcmVkT3B0aW1pemVkRm9yU2l6" + 
-        "ZRIJCgF4GAEgAigFIloKHFRlc3RPcHRpb25hbE9wdGltaXplZEZvclNpemUS" + 
-        "OgoBbxgBIAEoCzIvLnByb3RvYnVmX3VuaXR0ZXN0LlRlc3RSZXF1aXJlZE9w" + 
-        "dGltaXplZEZvclNpemVCR0gCguIJIUdvb2dsZS5Qcm90b2NvbEJ1ZmZlcnMu" + 
-        "VGVzdFByb3Rvc4riCRxVbml0VGVzdE9wdGltaXplRm9yUHJvdG9GaWxl"),
+        "Rm9yU2l6ZRjSCSABKAUycgoPdGVzdF9leHRlbnNpb24yEicucHJvdG9idWZf" + 
+        "dW5pdHRlc3QuVGVzdE9wdGltaXplZEZvclNpemUY0wkgASgLMi8ucHJvdG9i" + 
+        "dWZfdW5pdHRlc3QuVGVzdFJlcXVpcmVkT3B0aW1pemVkRm9yU2l6ZSIpChxU" + 
+        "ZXN0UmVxdWlyZWRPcHRpbWl6ZWRGb3JTaXplEgkKAXgYASACKAUiWgocVGVz" + 
+        "dE9wdGlvbmFsT3B0aW1pemVkRm9yU2l6ZRI6CgFvGAEgASgLMi8ucHJvdG9i" + 
+        "dWZfdW5pdHRlc3QuVGVzdFJlcXVpcmVkT3B0aW1pemVkRm9yU2l6ZUJHSAKC" + 
+        "4gkhR29vZ2xlLlByb3RvY29sQnVmZmVycy5UZXN0UHJvdG9ziuIJHFVuaXRU" + 
+        "ZXN0T3B0aW1pemVGb3JQcm90b0ZpbGU="),
         new pbd::FileDescriptor[] {
           global::Google.ProtocolBuffers.DescriptorProtos.CSharpOptions.Descriptor, 
           global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProtoFile.Descriptor, 
@@ -76,6 +79,8 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public static readonly pb::GeneratedExtensionBase<int> TestExtension =
         pb::GeneratedSingleExtension<int>.CreateInstance(Descriptor.Extensions[0]);
+    public static readonly pb::GeneratedExtensionBase<global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize> TestExtension2 =
+        pb::GeneratedSingleExtension<global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize>.CreateInstance(Descriptor.Extensions[1]);
     private bool hasI;
     private int i_ = 0;
     public bool HasI {
diff --git a/src/ProtocolBuffers/GeneratedBuilder.cs b/src/ProtocolBuffers/GeneratedBuilder.cs
index 4f577a790f..2005d9ee21 100644
--- a/src/ProtocolBuffers/GeneratedBuilder.cs
+++ b/src/ProtocolBuffers/GeneratedBuilder.cs
@@ -113,7 +113,7 @@ namespace Google.ProtocolBuffers {
         throw new ArgumentException("Message type mismatch");
       }
 
-      foreach (KeyValuePair<FieldDescriptor, object> entry in AllFields) {
+      foreach (KeyValuePair<FieldDescriptor, object> entry in other.AllFields) {
         FieldDescriptor field = entry.Key;
         if (field.IsRepeated) {
           // Concatenate repeated fields