diff --git a/synapse/replication/slave/storage/deviceinbox.py b/synapse/replication/slave/storage/deviceinbox.py
index cc860f9f9..f9102e0d8 100644
--- a/synapse/replication/slave/storage/deviceinbox.py
+++ b/synapse/replication/slave/storage/deviceinbox.py
@@ -17,6 +17,7 @@ from ._base import BaseSlavedStore
 from ._slaved_id_tracker import SlavedIdTracker
 from synapse.storage import DataStore
 from synapse.util.caches.stream_change_cache import StreamChangeCache
+from synapse.util.caches.expiringcache import ExpiringCache
 
 
 class SlavedDeviceInboxStore(BaseSlavedStore):
@@ -34,6 +35,13 @@ class SlavedDeviceInboxStore(BaseSlavedStore):
             self._device_inbox_id_gen.get_current_token()
         )
 
+        self._last_device_delete_cache = ExpiringCache(
+            cache_name="last_device_delete_cache",
+            clock=self._clock,
+            max_len=10000,
+            expiry_ms=30 * 60 * 1000,
+        )
+
     get_to_device_stream_token = DataStore.get_to_device_stream_token.__func__
     get_new_messages_for_device = DataStore.get_new_messages_for_device.__func__
     get_new_device_msgs_for_remote = DataStore.get_new_device_msgs_for_remote.__func__