diff --git a/pymongo/server_description.py b/pymongo/server_description.py
index e2b29bca..2c8ccdeb 100644
--- a/pymongo/server_description.py
+++ b/pymongo/server_description.py
@@ -83,6 +83,9 @@ class ServerDescription(object):
         else:
             self._last_write_date = None
 
+        if self.is_server_type_known:
+            assert self._round_trip_time is not None
+
     @property
     def address(self):
         """The address (host, port) of this server."""