diff --git a/changelog.d/15770.bugfix b/changelog.d/15770.bugfix
new file mode 100644
index 000000000..a65340d38
--- /dev/null
+++ b/changelog.d/15770.bugfix
@@ -0,0 +1 @@
+Fix requesting multiple keys at once over federation, related to [MSC3983](https://github.com/matrix-org/matrix-spec-proposals/pull/3983).
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 9425b3250..61fa3b30a 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -1016,7 +1016,9 @@ class FederationServer(FederationBase):
             for user_id, device_keys in result.items():
                 for device_id, keys in device_keys.items():
                     for key_id, key in keys.items():
-                        json_result.setdefault(user_id, {})[device_id] = {key_id: key}
+                        json_result.setdefault(user_id, {}).setdefault(device_id, {})[
+                            key_id
+                        ] = key
 
         logger.info(
             "Claimed one-time-keys: %s",