diff --git a/transferwee.py b/transferwee.py
index 8833265..49152db 100755
--- a/transferwee.py
+++ b/transferwee.py
@@ -378,7 +378,10 @@ def _storm_prepare(authorization: str, filenames: List[str]) -> Dict[Any, Any]:
     files_bids = [_storm_prepare_item(f) for f in filenames]
 
     blocks = [i for sublist in files_bids for i in sublist]
-    response = {"ok": True, "data": {"blocks": []}}
+    response: Dict[str, Union[bool, Dict[str, List[Any]]]] = {
+        "ok": True,
+        "data": {"blocks": []},
+    }
     chunk_size = 100
     for i in range(0, len(blocks), chunk_size):
         j = {
@@ -406,7 +409,9 @@ def _storm_prepare(authorization: str, filenames: List[str]) -> Dict[Any, Any]:
         if not r_json["ok"]:
             logger.error(r_json)
         response["ok"] = response["ok"] and r_json["ok"]
-        response["data"]["blocks"] += r_json["data"]["blocks"]
+        typing.cast(typing.Dict[str, List], response["data"])[
+            "blocks"
+        ] += typing.cast(typing.Dict[str, List], r_json["data"])["blocks"]
 
     return {"files_bids": files_bids, "blocks": response}