diff --git a/synapse/storage/database.py b/synapse/storage/database.py
index 207fa9d7df..53858db9a3 100644
--- a/synapse/storage/database.py
+++ b/synapse/storage/database.py
@@ -1120,7 +1120,7 @@ class DatabasePool(object):
         txn: LoggingTransaction,
         table: str,
         keyvalues: Dict[str, Any],
-        retcol: Iterable[str],
+        retcol: str,
     ) -> List[Any]:
         sql = ("SELECT %(retcol)s FROM %(table)s") % {"retcol": retcol, "table": table}