diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index d4dbfd3dca..54c25dfe3e 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -1000,18 +1000,12 @@ class FederationServer(FederationBase):
         self, query: List[Tuple[str, str, str, int]], always_include_fallback_keys: bool
     ) -> Dict[str, Any]:
         log_kv({"message": "Claiming one time keys.", "user, device pairs": query})
-        results = await self._e2e_keys_handler.claim_local_one_time_keys(
-            query, always_include_fallback_keys=always_include_fallback_keys
-        )
-
         json_result: Dict[str, Dict[str, Dict[str, JsonSerializable]]] = {}
-        for result in results:
-            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, {}).setdefault(device_id, {})[
-                            key_id
-                        ] = key
+        await self._e2e_keys_handler.claim_local_one_time_keys(
+            query,
+            always_include_fallback_keys=always_include_fallback_keys,
+            result_dict=json_result,
+        )
 
         logger.info(
             "Claimed one-time-keys: %s",
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 9e51a34d70..5c356b0cff 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -561,7 +561,8 @@ class E2eKeysHandler:
         self,
         local_query: List[Tuple[str, str, str, int]],
         always_include_fallback_keys: bool,
-    ) -> Iterable[Mapping[str, Mapping[str, Mapping[str, JsonSerializable]]]]:
+        result_dict: Dict[str, Dict[str, Dict[str, JsonSerializable]]],
+    ) -> None:
         """Claim one time keys for local users.
 
         1. Attempt to claim OTKs from the database.
@@ -571,11 +572,20 @@ class E2eKeysHandler:
         Args:
             local_query: An iterable of tuples of (user ID, device ID, algorithm).
             always_include_fallback_keys: True to always include fallback keys.
-
-        Returns:
-            An iterable of maps of user ID -> a map device ID -> a map of key ID -> key.
+            result_dict: A dict to update with the results.
+                {user_id -> { device_id -> { key_id -> key string/object }}}
         """
 
+        def update_result_dict(
+            results: Mapping[str, Mapping[str, Mapping[str, JsonSerializable]]]
+        ) -> None:
+            """Stash results from a store query in `result_dict`"""
+            for user_id, device_keys in results.items():
+                user_result_dict = result_dict.setdefault(user_id, {})
+                for device_id, keys in device_keys.items():
+                    device_result_dict = user_result_dict.setdefault(device_id, {})
+                    device_result_dict.update(keys)
+
         # Cap the number of OTKs that can be claimed at once to avoid abuse.
         local_query = [
             (user_id, device_id, algorithm, min(count, 5))
@@ -583,6 +593,7 @@ class E2eKeysHandler:
         ]
 
         otk_results, not_found = await self.store.claim_e2e_one_time_keys(local_query)
+        update_result_dict(otk_results)
 
         # If the application services have not provided any keys via the C-S
         # API, query it directly for one-time keys.
@@ -593,6 +604,7 @@ class E2eKeysHandler:
                 appservice_results,
                 not_found,
             ) = await self._appservice_handler.claim_e2e_one_time_keys(not_found)
+            update_result_dict(appservice_results)
         else:
             appservice_results = {}
 
@@ -647,10 +659,7 @@ class E2eKeysHandler:
         # For each user that does not have a one-time keys available, see if
         # there is a fallback key.
         fallback_results = await self.store.claim_e2e_fallback_keys(fallback_query)
-
-        # Return the results in order, each item from the input query should
-        # only appear once in the combined list.
-        return (otk_results, appservice_results, fallback_results)
+        update_result_dict(fallback_results)
 
     @trace
     async def claim_one_time_keys(
@@ -676,19 +685,11 @@ class E2eKeysHandler:
         set_tag("local_key_query", str(local_query))
         set_tag("remote_key_query", str(remote_queries))
 
-        results = await self.claim_local_one_time_keys(
-            local_query, always_include_fallback_keys
-        )
-
         # A map of user ID -> device ID -> key ID -> key.
         json_result: Dict[str, Dict[str, Dict[str, JsonSerializable]]] = {}
-        for result in results:
-            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, {}).setdefault(
-                            device_id, {}
-                        ).update({key_id: key})
+        await self.claim_local_one_time_keys(
+            local_query, always_include_fallback_keys, json_result
+        )
 
         # Remote failures.
         failures: Dict[str, JsonDict] = {}