diff --git a/apitools/base/py/encoding.py b/apitools/base/py/encoding.py
index 26deb23c..ec7518cb 100644
--- a/apitools/base/py/encoding.py
+++ b/apitools/base/py/encoding.py
@@ -88,10 +88,9 @@ def Register(field_type):
     return Register
 
 
-# TODO(craigcitro): Delete this function with the switch to proto2.
 def CopyProtoMessage(message):
-    codec = protojson.ProtoJson()
-    return codec.decode_message(type(message), codec.encode_message(message))
+    """Make a deep copy of a message."""
+    return JsonToMessage(type(message), MessageToJson(message))
 
 
 def MessageToJson(message, include_fields=None):
@@ -438,12 +437,19 @@ def _DecodeUnrecognizedFields(message, pair_type):
     return new_values
 
 
+def _CopyProtoMessageVanillaProtoJson(message):
+    codec = protojson.ProtoJson()
+    return codec.decode_message(type(message), codec.encode_message(message))
+
+
 def _EncodeUnknownFields(message):
     """Remap unknown fields in message out of message.source."""
     source = _UNRECOGNIZED_FIELD_MAPPINGS.get(type(message))
     if source is None:
         return message
-    result = CopyProtoMessage(message)
+    # CopyProtoMessage uses _ProtoJsonApiTools, which uses this message. Use
+    # the vanilla protojson-based copy function to avoid infinite recursion.
+    result = _CopyProtoMessageVanillaProtoJson(message)
     pairs_field = message.field_by_name(source)
     if not isinstance(pairs_field, messages.MessageField):
         raise exceptions.InvalidUserInputError(
diff --git a/apitools/base/py/encoding_test.py b/apitools/base/py/encoding_test.py
index d73b1917..682d4567 100644
--- a/apitools/base/py/encoding_test.py
+++ b/apitools/base/py/encoding_test.py
@@ -188,6 +188,26 @@ def testCopyProtoMessage(self):
         msg.field = 'def'
         self.assertNotEqual(msg.field, new_msg.field)
 
+    def testCopyProtoMessageInvalidEnum(self):
+        json_msg = '{"field_one": "BAD_VALUE"}'
+        orig_msg = encoding.JsonToMessage(MessageWithEnum, json_msg)
+        new_msg = encoding.CopyProtoMessage(orig_msg)
+        for msg in (orig_msg, new_msg):
+            self.assertEqual(msg.all_unrecognized_fields(), ['field_one'])
+            self.assertEqual(
+                msg.get_unrecognized_field_info('field_one',
+                                                value_default=None),
+                ('BAD_VALUE', messages.Variant.ENUM))
+
+    def testCopyProtoMessageAdditionalProperties(self):
+        msg = AdditionalPropertiesMessage(additionalProperties=[
+            AdditionalPropertiesMessage.AdditionalProperty(
+                key='key', value='value')])
+        new_msg = encoding.CopyProtoMessage(msg)
+        self.assertEqual(len(new_msg.additionalProperties), 1)
+        self.assertEqual(new_msg.additionalProperties[0].key, 'key')
+        self.assertEqual(new_msg.additionalProperties[0].value, 'value')
+
     def testBytesEncoding(self):
         b64_str = 'AAc+'
         b64_msg = '{"field": "%s"}' % b64_str