1 files changed, 1 insertions, 0 deletions
diff --git a/synapse/util/file_consumer.py b/synapse/util/file_consumer.py
index de2adacd70..46771a401b 100644
--- a/synapse/util/file_consumer.py
+++ b/synapse/util/file_consumer.py
@@ -142,6 +142,7 @@ class BackgroundFileConsumer:
 
     def wait(self) -> "Deferred[None]":
         """Returns a deferred that resolves when finished writing to file"""
+        assert self._finished_deferred is not None
         return make_deferred_yieldable(self._finished_deferred)
 
     def _resume_paused_producer(self) -> None: