diff --git a/tests/server.py b/tests/server.py
index 017e5cf63..b29df3759 100644
--- a/tests/server.py
+++ b/tests/server.py
@@ -850,4 +850,7 @@ def setup_test_homeserver(
 
     hs.get_auth_handler().validate_hash = validate_hash
 
+    # Make the threadpool and database transactions synchronous for testing.
+    make_test_homeserver_synchronous(hs)
+
     return hs