diff --git a/synapse/storage/database.py b/synapse/storage/database.py
index 0bbaa9e89835..90721c117f0f 100644
--- a/synapse/storage/database.py
+++ b/synapse/storage/database.py
@@ -1818,7 +1818,7 @@ def simple_update_many_txn(
         key_names: Collection[str],
         key_values: Collection[Iterable[Any]],
         value_names: Collection[str],
-        value_values: Iterable[Iterable[Any]],
+        value_values: Collection[Iterable[Any]],
     ) -> None:
         """
         Update, many times, using batching where possible.
@@ -1831,6 +1831,11 @@ def simple_update_many_txn(
             value_values: A list of each row's value column values.
         """
 
+        if len(value_values) != len(key_values):
+            raise ValueError(
+                f"{len(key_values)} key rows and {len(value_values)} value rows: should be the same number."
+            )
+
         # List of value names, then key names
         allnames: List[str] = []
         allnames.extend(value_names)