diff --git a/changelog.d/6385.bugfix b/changelog.d/6385.bugfix
new file mode 100644
index 000000000..7a2bc0217
--- /dev/null
+++ b/changelog.d/6385.bugfix
@@ -0,0 +1 @@
+Prevent error on trying to search a upgraded room when the server is not in the predecessor room.
\ No newline at end of file
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index c0dcf9abf..13865c470 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1299,7 +1299,7 @@ class FederationHandler(BaseHandler):
             # Check whether this room is the result of an upgrade of a room we already know
             # about. If so, migrate over user information
             predecessor = yield self.store.get_room_predecessor(room_id)
-            if not predecessor:
+            if not predecessor or not isinstance(predecessor.get("room_id"), str):
                 return
             old_room_id = predecessor["room_id"]
             logger.debug(
@@ -1542,7 +1542,7 @@ class FederationHandler(BaseHandler):
     @defer.inlineCallbacks
     def do_remotely_reject_invite(self, target_hosts, room_id, user_id, content):
         origin, event, event_format_version = yield self._make_and_verify_event(
-            target_hosts, room_id, user_id, "leave", content=content,
+            target_hosts, room_id, user_id, "leave", content=content
         )
         # Mark as outlier as we don't have any state for this event; we're not
         # even in the room.
diff --git a/synapse/handlers/search.py b/synapse/handlers/search.py
index 56ed262a1..ef750d149 100644
--- a/synapse/handlers/search.py
+++ b/synapse/handlers/search.py
@@ -21,7 +21,7 @@ from unpaddedbase64 import decode_base64, encode_base64
 from twisted.internet import defer
 
 from synapse.api.constants import EventTypes, Membership
-from synapse.api.errors import SynapseError
+from synapse.api.errors import NotFoundError, SynapseError
 from synapse.api.filtering import Filter
 from synapse.storage.state import StateFilter
 from synapse.visibility import filter_events_for_client
@@ -37,6 +37,7 @@ class SearchHandler(BaseHandler):
         self._event_serializer = hs.get_event_client_serializer()
         self.storage = hs.get_storage()
         self.state_store = self.storage.state
+        self.auth = hs.get_auth()
 
     @defer.inlineCallbacks
     def get_old_rooms_from_upgraded_room(self, room_id):
@@ -53,23 +54,38 @@ class SearchHandler(BaseHandler):
             room_id (str): id of the room to search through.
 
         Returns:
-            Deferred[iterable[unicode]]: predecessor room ids
+            Deferred[iterable[str]]: predecessor room ids
         """
 
         historical_room_ids = []
 
-        while True:
-            predecessor = yield self.store.get_room_predecessor(room_id)
+        # The initial room must have been known for us to get this far
+        predecessor = yield self.store.get_room_predecessor(room_id)
 
-            # If no predecessor, assume we've hit a dead end
+        while True:
             if not predecessor:
+                # We have reached the end of the chain of predecessors
                 break
 
-            # Add predecessor's room ID
-            historical_room_ids.append(predecessor["room_id"])
+            if not isinstance(predecessor.get("room_id"), str):
+                # This predecessor object is malformed. Exit here
+                break
 
-            # Scan through the old room for further predecessors
-            room_id = predecessor["room_id"]
+            predecessor_room_id = predecessor["room_id"]
+
+            # Don't add it to the list until we have checked that we are in the room
+            try:
+                next_predecessor_room = yield self.store.get_room_predecessor(
+                    predecessor_room_id
+                )
+            except NotFoundError:
+                # The predecessor is not a known room, so we are done here
+                break
+
+            historical_room_ids.append(predecessor_room_id)
+
+            # And repeat
+            predecessor = next_predecessor_room
 
         return historical_room_ids
 
diff --git a/synapse/storage/data_stores/main/state.py b/synapse/storage/data_stores/main/state.py
index 9ef7b48c7..dcc6b43cd 100644
--- a/synapse/storage/data_stores/main/state.py
+++ b/synapse/storage/data_stores/main/state.py
@@ -278,7 +278,7 @@ class StateGroupWorkerStore(
 
     @defer.inlineCallbacks
     def get_room_predecessor(self, room_id):
-        """Get the predecessor room of an upgraded room if one exists.
+        """Get the predecessor of an upgraded room if it exists.
         Otherwise return None.
 
         Args:
@@ -291,14 +291,22 @@ class StateGroupWorkerStore(
                     * room_id (str): The room ID of the predecessor room
                     * event_id (str): The ID of the tombstone event in the predecessor room
 
+                None if a predecessor key is not found, or is not a dictionary.
+
         Raises:
-            NotFoundError if the room is unknown
+            NotFoundError if the given room is unknown
         """
         # Retrieve the room's create event
         create_event = yield self.get_create_event_for_room(room_id)
 
-        # Return predecessor if present
-        return create_event.content.get("predecessor", None)
+        # Retrieve the predecessor key of the create event
+        predecessor = create_event.content.get("predecessor", None)
+
+        # Ensure the key is a dictionary
+        if not isinstance(predecessor, dict):
+            return None
+
+        return predecessor
 
     @defer.inlineCallbacks
     def get_create_event_for_room(self, room_id):
@@ -318,7 +326,7 @@ class StateGroupWorkerStore(
 
         # If we can't find the create event, assume we've hit a dead end
         if not create_id:
-            raise NotFoundError("Unknown room %s" % (room_id))
+            raise NotFoundError("Unknown room %s" % (room_id,))
 
         # Retrieve the room's create event and return
         create_event = yield self.get_event(create_id)