diff --git a/third_party/2/concurrent/futures/__init__.pyi b/third_party/2/concurrent/futures/__init__.pyi
index 492ee1bfa..5e5888876 100644
--- a/third_party/2/concurrent/futures/__init__.pyi
+++ b/third_party/2/concurrent/futures/__init__.pyi
@@ -1,7 +1,11 @@
-from typing import TypeVar, Generic, Any, Iterable, Iterator, Callable, Tuple, Union
+from typing import TypeVar, Generic, Any, Iterable, Iterator, Callable, Optional, Set, Tuple, Union
 
 _T = TypeVar('_T')
 
+class Error(Exception): ...
+class CancelledError(Error): ...
+class TimeoutError(Error): ...
+
 class Future(Generic[_T]):
     def cancel(self) -> bool: ...
     def cancelled(self) -> bool: ...
@@ -28,7 +32,7 @@ class ThreadPoolExecutor(Executor):
 class ProcessPoolExecutor(Executor):
     def __init__(self, max_workers: Union[int, None] = ...) -> None: ...
 
-def wait(fs: Iterable[Future], timeout: float = ..., return_when: str = ...) -> Tuple[Iterable[Future], Iterable[Future]]: ...
+def wait(fs: Iterable[Future], timeout: Optional[float] = ..., return_when: str = ...) -> Tuple[Set[Future], Set[Future]]: ...
 
 FIRST_COMPLETED = ...  # type: str
 FIRST_EXCEPTION = ...  # type: str