diff --git a/python/google/protobuf/internal/json_format_test.py b/python/google/protobuf/internal/json_format_test.py
index 4310115a40..672a9b424d 100755
--- a/python/google/protobuf/internal/json_format_test.py
+++ b/python/google/protobuf/internal/json_format_test.py
@@ -1119,6 +1119,30 @@ class JsonFormatTest(JsonFormatBase):
         'Failed to parse value field: Struct must be in a dict which is 1234',
         json_format.Parse, text, message)
 
+  def testTimestampInvalidStringValue(self):
+    message = json_format_proto3_pb2.TestTimestamp()
+    text = '{"value": {"foo": 123}}'
+    self.assertRaisesRegexp(
+        json_format.ParseError,
+        r"Timestamp JSON value not a string: {u?'foo': 123}",
+        json_format.Parse, text, message)
+
+  def testDurationInvalidStringValue(self):
+    message = json_format_proto3_pb2.TestDuration()
+    text = '{"value": {"foo": 123}}'
+    self.assertRaisesRegexp(
+        json_format.ParseError,
+        r"Duration JSON value not a string: {u?'foo': 123}",
+        json_format.Parse, text, message)
+
+  def testFieldMaskInvalidStringValue(self):
+    message = json_format_proto3_pb2.TestFieldMask()
+    text = '{"value": {"foo": 123}}'
+    self.assertRaisesRegexp(
+        json_format.ParseError,
+        r"FieldMask JSON value not a string: {u?'foo': 123}",
+        json_format.Parse, text, message)
+
   def testInvalidAny(self):
     message = any_pb2.Any()
     text = '{"@type": "type.googleapis.com/google.protobuf.Int32Value"}'
@@ -1217,7 +1241,7 @@ class JsonFormatTest(JsonFormatBase):
   def testParseDictUnknownValueType(self):
     class UnknownClass(object):
 
-      def __str__(self):
+      def __repr__(self):
         return 'v'
     message = json_format_proto3_pb2.TestValue()
     self.assertRaisesRegexp(
diff --git a/python/google/protobuf/internal/well_known_types.py b/python/google/protobuf/internal/well_known_types.py
index 6f55d6b17b..56659ac7e9 100644
--- a/python/google/protobuf/internal/well_known_types.py
+++ b/python/google/protobuf/internal/well_known_types.py
@@ -143,6 +143,9 @@ class Timestamp(object):
     Raises:
       ValueError: On parsing problems.
     """
+    if not isinstance(value, six.string_types):
+      raise ValueError(
+          'Timestamp JSON value not a string: {!r}'.format(value))
     timezone_offset = value.find('Z')
     if timezone_offset == -1:
       timezone_offset = value.find('+')
@@ -303,6 +306,9 @@ class Duration(object):
     Raises:
       ValueError: On parsing problems.
     """
+    if not isinstance(value, six.string_types):
+      raise ValueError(
+          'Duration JSON value not a string: {!r}'.format(value))
     if len(value) < 1 or value[-1] != 's':
       raise ValueError(
           'Duration must end with letter "s": {0}.'.format(value))
@@ -428,6 +434,9 @@ class FieldMask(object):
 
   def FromJsonString(self, value):
     """Converts string to FieldMask according to proto3 JSON spec."""
+    if not isinstance(value, six.string_types):
+      raise ValueError(
+          'FieldMask JSON value not a string: {!r}'.format(value))
     self.Clear()
     if value:
       for path in value.split(','):