diff --git a/oauthlib/oauth2/rfc6749/grant_types/refresh_token.py b/oauthlib/oauth2/rfc6749/grant_types/refresh_token.py
index ce33df0e..43bf55ac 100644
--- a/oauthlib/oauth2/rfc6749/grant_types/refresh_token.py
+++ b/oauthlib/oauth2/rfc6749/grant_types/refresh_token.py
@@ -101,6 +101,9 @@ def validate_token_request(self, request):
             if not self.request_validator.authenticate_client(request):
                 log.debug('Invalid client (%r), denying access.', request)
                 raise errors.InvalidClientError(request=request)
+            # Ensure that request.client_id is set.
+            if request.client_id is None and request.client is not None:
+                request.client_id = request.client.client_id
         elif not self.request_validator.authenticate_client_id(request.client_id, request):
             log.debug('Client authentication failed, %r.', request)
             raise errors.InvalidClientError(request=request)
diff --git a/tests/oauth2/rfc6749/grant_types/test_refresh_token.py b/tests/oauth2/rfc6749/grant_types/test_refresh_token.py
index 581f2a4d..0a4ddd9a 100644
--- a/tests/oauth2/rfc6749/grant_types/test_refresh_token.py
+++ b/tests/oauth2/rfc6749/grant_types/test_refresh_token.py
@@ -130,6 +130,22 @@ def test_authentication_required(self):
                           self.request)
         self.mock_validator.client_authentication_required.assert_called_once_with(self.request)
 
+
+    def test_authentication_required_populate_client_id(self):
+        """
+        Make sure that request.client_id is populated from
+        request.client.client_id if None.
+
+        """
+        self.mock_validator.client_authentication_required.return_value = True
+        self.mock_validator.authenticate_client.return_value = True
+        # self.mock_validator.authenticate_client_id.return_value = False
+        # self.request.code = 'waffles'
+        self.request.client_id = None
+        self.request.client.client_id = 'foobar'
+        self.auth.validate_token_request(self.request)
+        self.request.client_id = 'foobar'
+
     def test_invalid_grant_type(self):
         self.request.grant_type = 'wrong_type'
         self.assertRaises(errors.UnsupportedGrantTypeError,