diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index c14ddac69..79b6ccf66 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -683,7 +683,7 @@ class FederationHandler(BaseHandler, FederationBase):
     async def _validate_forwarded_event(self, event: EventBase) -> Tuple[bool, Optional[str]]:
         try:
             source_evt_dict = {**event.content[self._forwarded_key]}
-            room_version = source_evt_dict["unsigned"]["room_version"]
+            room_version_identifier = source_evt_dict["unsigned"]["room_version"]
             source_evt_dict["type"] = event.type
             source_evt_dict["content"] = {**event.content}
             del source_evt_dict["unsigned"]
@@ -691,11 +691,18 @@ class FederationHandler(BaseHandler, FederationBase):
         except (KeyError, TypeError):
             logger.exception("Failed to read forward data")
             return False, None
+
+        room_version = KNOWN_ROOM_VERSIONS.get(room_version_identifier)
+        if not room_version:
+            # We don't support the source room version, so we can't verify the event :(
+            return False, None
+
         try:
             source_evt = event_from_pdu_json(source_evt_dict, room_version)
         except SynapseError:
             logger.exception("Failed to parse forward data")
             return False, None
+
         try:
             checked_evt = await self._check_sigs_and_hash(room_version, source_evt)
             # _check_sigs_and_hash returns a redacted event if hash validation failed and
diff --git a/synapse/rest/client/v2_alpha/forward_event.py b/synapse/rest/client/v2_alpha/forward_event.py
index d77c412f1..8fbae6597 100644
--- a/synapse/rest/client/v2_alpha/forward_event.py
+++ b/synapse/rest/client/v2_alpha/forward_event.py
@@ -85,8 +85,9 @@ class RoomEventForwardServlet(RestServlet):
                                errcode=self._err_not_forwardable)
         elif not has_forward_meta:
             content[self._data_key] = event_dict
+            room_version = await self.store.get_room_version(event.room_id)
             content[self._data_key]["unsigned"] = {
-                "room_version": await self.store.get_room_version(event.room_id),
+                "room_version": room_version.identifier,
                 # TODO add sender profile info here
             }