diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index 2e9c6b688b..22aab9b0a8 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -37,7 +37,7 @@ class DirectoryHandler(BaseHandler):
         )
 
     @defer.inlineCallbacks
-    def create_association(self, room_alias, room_id, servers):
+    def create_association(self, room_alias, room_id, servers=None):
         # TODO(erikj): Do auth.
 
         if not room_alias.is_mine:
@@ -48,6 +48,12 @@ class DirectoryHandler(BaseHandler):
 
         # TODO(erikj): Check if there is a current association.
 
+        if not servers:
+            servers = yield self.store.get_joined_hosts_for_room(room_id)
+
+        if not servers:
+            raise SynapseError(400, "Failed to get server list")
+
         yield self.store.create_room_alias_association(
             room_alias,
             room_id,
@@ -83,6 +89,9 @@ class DirectoryHandler(BaseHandler):
             defer.returnValue({})
             return
 
+        extra_servers = yield self.store.get_joined_hosts_for_room(room_id)
+        servers = list(set(extra_servers) | set(servers))
+
         defer.returnValue({
             "room_id": room_id,
             "servers": servers,
diff --git a/synapse/rest/directory.py b/synapse/rest/directory.py
index dc347652a0..29ace4f1d3 100644
--- a/synapse/rest/directory.py
+++ b/synapse/rest/directory.py
@@ -54,7 +54,7 @@ class ClientDirectoryServer(RestServlet):
         logger.debug("Got room name: %s", room_alias.to_string())
 
         room_id = content["room_id"]
-        servers = content["servers"]
+        servers = content["servers"] if "servers" in content else None
 
         logger.debug("Got room_id: %s", room_id)
         logger.debug("Got servers: %s", servers)
diff --git a/tests/handlers/test_directory.py b/tests/handlers/test_directory.py
index e509c1b629..2361c6d031 100644
--- a/tests/handlers/test_directory.py
+++ b/tests/handlers/test_directory.py
@@ -50,6 +50,7 @@ class DirectoryTestCase(unittest.TestCase):
         hs = HomeServer("test",
             datastore=Mock(spec=[
                 "get_association_from_room_alias",
+                "get_joined_hosts_for_room",
             ]),
             http_client=None,
             resource_for_federation=Mock(),
@@ -61,6 +62,10 @@ class DirectoryTestCase(unittest.TestCase):
 
         self.datastore = hs.get_datastore()
 
+        def hosts(room_id):
+            return defer.succeed([])
+        self.datastore.get_joined_hosts_for_room.side_effect = hosts
+
         self.my_room = hs.parse_roomalias("#my-room:test")
         self.remote_room = hs.parse_roomalias("#another:remote")