diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index a1c289b24a..5415b0c9ee 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -48,7 +48,7 @@ from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.replication.http.send_event import ReplicationSendEventRestServlet
 from synapse.storage.data_stores.main.events_worker import EventRedactBehaviour
 from synapse.storage.state import StateFilter
-from synapse.types import RoomAlias, UserID, create_requester
+from synapse.types import Collection, RoomAlias, UserID, create_requester
 from synapse.util.async_helpers import Linearizer
 from synapse.util.frozenutils import frozendict_json_encoder
 from synapse.util.metrics import measure_func
@@ -497,10 +497,14 @@ class EventCreationHandler(object):
         if txn_id is not None:
             builder.internal_metadata.txn_id = txn_id
 
+        prev_event_ids = (
+            None
+            if prev_events_and_hashes is None
+            else [event_id for event_id, _, _ in prev_events_and_hashes]
+        )
+
         event, context = yield self.create_new_client_event(
-            builder=builder,
-            requester=requester,
-            prev_events_and_hashes=prev_events_and_hashes,
+            builder=builder, requester=requester, prev_event_ids=prev_event_ids,
         )
 
         # In an ideal world we wouldn't need the second part of this condition. However,
@@ -714,7 +718,7 @@ class EventCreationHandler(object):
     @measure_func("create_new_client_event")
     @defer.inlineCallbacks
     def create_new_client_event(
-        self, builder, requester=None, prev_events_and_hashes=None
+        self, builder, requester=None, prev_event_ids: Optional[Collection[str]] = None
     ):
         """Create a new event for a local client
 
@@ -723,10 +727,9 @@ class EventCreationHandler(object):
 
             requester (synapse.types.Requester|None):
 
-            prev_events_and_hashes (list[(str, dict[str, str], int)]|None):
+            prev_event_ids:
                 the forward extremities to use as the prev_events for the
-                new event. For each event, a tuple of (event_id, hashes, depth)
-                where *hashes* is a map from algorithm to hash.
+                new event.
 
                 If None, they will be requested from the database.
 
@@ -734,12 +737,11 @@ class EventCreationHandler(object):
             Deferred[(synapse.events.EventBase, synapse.events.snapshot.EventContext)]
         """
 
-        if prev_events_and_hashes is not None:
-            assert len(prev_events_and_hashes) <= 10, (
+        if prev_event_ids is not None:
+            assert len(prev_event_ids) <= 10, (
                 "Attempting to create an event with %i prev_events"
-                % (len(prev_events_and_hashes),)
+                % (len(prev_event_ids),)
             )
-            prev_event_ids = [event_id for event_id, _, _ in prev_events_and_hashes]
         else:
             prev_event_ids = yield self.store.get_prev_events_for_room(builder.room_id)
 
diff --git a/synapse/types.py b/synapse/types.py
index aafc3ffe74..cd996c0b5a 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -15,6 +15,7 @@
 # limitations under the License.
 import re
 import string
+import sys
 from collections import namedtuple
 
 import attr
@@ -23,6 +24,17 @@ from unpaddedbase64 import decode_base64
 
 from synapse.api.errors import SynapseError
 
+# define a version of typing.Collection that works on python 3.5
+if sys.version_info[:3] >= (3, 6, 0):
+    from typing import Collection
+else:
+    from typing import Sized, Iterable, Container, TypeVar
+
+    T_co = TypeVar("T_co", covariant=True)
+
+    class Collection(Iterable[T_co], Container[T_co], Sized):
+        __slots__ = ()
+
 
 class Requester(
     namedtuple(