diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 88546ad614f0..3bb938166349 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -16,6 +16,7 @@
 # limitations under the License.
 import logging
 import random
+from abc import ABCMeta
 
 from six import PY2
 from six.moves import builtins
@@ -30,7 +31,8 @@
 logger = logging.getLogger(__name__)
 
 
-class SQLBaseStore(object):
+# some of our subclasses have abstract methods, so we use the ABCMeta metaclass.
+class SQLBaseStore(metaclass=ABCMeta):
     """Base class for data stores that holds helper functions.
 
     Note that multiple instances of this class will exist as there will be one
diff --git a/synapse/storage/data_stores/main/room.py b/synapse/storage/data_stores/main/room.py
index 0cf932282e9b..79cfd39194c4 100644
--- a/synapse/storage/data_stores/main/room.py
+++ b/synapse/storage/data_stores/main/room.py
@@ -17,6 +17,7 @@
 import collections
 import logging
 import re
+from abc import abstractmethod
 from typing import Optional, Tuple
 
 from six import integer_types
@@ -499,7 +500,12 @@ def _get_rooms(txn):
 
         return len(rooms)
 
+    @abstractmethod
     def set_room_is_public(self, room_id, is_public):
+        # this will need to be implemented if a background update is performed with
+        # existing (tombstoned, public) rooms in the database.
+        #
+        # It's overridden by RoomStore for the synapse master.
         raise NotImplementedError()