diff --git a/cloud/cost.py b/cloud/cost.py
index 308be1b..0033a71 100644
--- a/cloud/cost.py
+++ b/cloud/cost.py
@@ -536,6 +536,12 @@ class CostStore:
             raise ValueError(
                 "exactly one of task_id, project_id, creator_id required")
         join = ""
+        # 类型标注而已（A6）：首分支会把 params 推断成单元素元组，
+        # 导致下面 params + (month,) 报 tuple[Any] vs tuple[Any, str]；
+        # 标注成裸 tuple（即 tuple[Any, ...]）后三分支与拼接均合法。
+        # 初值 () 永不被使用——if/elif/else 三分支必居其一重写它，
+        # 运行时语义不变。
+        params: tuple = ()
         if task_id is not None:
             where, params = "c.task_id = ?", (task_id,)
         elif project_id is not None: