diff --git a/apitools/base/py/encoding_helper.py b/apitools/base/py/encoding_helper.py
index 2d8a449..96e3ebd 100644
--- a/apitools/base/py/encoding_helper.py
+++ b/apitools/base/py/encoding_helper.py
@@ -691,7 +691,7 @@ def _EncodeCustomFieldNames(message, encoded_value):
     if field_remappings:
         decoded_value = json.loads(encoded_value)
         for python_name, json_name in field_remappings:
-            if python_name in encoded_value:
+            if python_name in decoded_value:
                 decoded_value[json_name] = decoded_value.pop(python_name)
         encoded_value = json.dumps(decoded_value)
     return encoded_value
diff --git a/apitools/base/py/encoding_test.py b/apitools/base/py/encoding_test.py
index a681670..2c21a50 100644
--- a/apitools/base/py/encoding_test.py
+++ b/apitools/base/py/encoding_test.py
@@ -205,6 +205,7 @@ class SomeEnum(messages.Enum):
     another_field = messages.StringField(3)
     repeated_enum = messages.EnumField(SomeEnum, 4, repeated=True)
     repeated_field = messages.StringField(5, repeated=True)
+    field = messages.StringField(6)
 
 
 class MessageWithPackageAndRemappings(messages.Message):
@@ -236,6 +237,8 @@ class AdditionalProperty(messages.Message):
                                    'another_field', 'anotherField')
 encoding.AddCustomJsonFieldMapping(MessageWithRemappings,
                                    'repeated_field', 'repeatedField')
+encoding.AddCustomJsonFieldMapping(MessageWithRemappings,
+                                   'field', 'remappedField')
 
 
 class EncodingTest(unittest.TestCase):
@@ -644,6 +647,19 @@ def testMultipleRemapping(self):
         self.assertEqual(
             msg, encoding.JsonToMessage(MessageWithRemappings, json_message))
 
+    def testSubstringFieldRemapping(self):
+        # 'field' is a substring of 'another_field' and 'repeated_field',
+        # as well as a substring of the string value below. When 'field' is
+        # unset, encoding must not raise KeyError.
+        msg = MessageWithRemappings(
+            another_field='value containing field substring')
+        json_message = encoding.MessageToJson(msg)
+        self.assertEqual(
+            '{"anotherField": "value containing field substring"}',
+            json_message)
+        self.assertEqual(
+            msg, encoding.JsonToMessage(MessageWithRemappings, json_message))
+
     def testRepeatedRemapping(self):
         # Should allow remapping if the mapping remains the same.
         encoding.AddCustomJsonEnumMapping(MessageWithRemappings.SomeEnum,