diff --git a/stdlib/3/itertools.pyi b/stdlib/3/itertools.pyi
index 59bd40b83..ddefb5af4 100644
--- a/stdlib/3/itertools.pyi
+++ b/stdlib/3/itertools.pyi
@@ -21,12 +21,12 @@ def repeat(object: _T, times: int) -> Iterator[_T]: ...
 
 if sys.version_info >= (3, 8):
     @overload
-    def accumulate(iterable: Iterable[_T], func: Callable[[_T, _T], _T] = ...) -> Iterator[_T]: ...
+    def accumulate(iterable: Iterable[_T], func: None = ..., *, initial: Optional[_T] = ...) -> Iterator[_T]: ...
     @overload
-    def accumulate(iterable: Iterable[_T], func: Callable[[_S, _T], _S], initial: Optional[_S]) -> Iterator[_S]: ...
+    def accumulate(iterable: Iterable[_T], func: Callable[[_S, _T], _S], *, initial: Optional[_S] = ...) -> Iterator[_S]: ...
 
 else:
-    def accumulate(iterable: Iterable[_T], func: Callable[[_T, _T], _T] = ...) -> Iterator[_T]: ...
+    def accumulate(iterable: Iterable[_T], func: Optional[Callable[[_T, _T], _T]] = ...) -> Iterator[_T]: ...
 
 class chain(Iterator[_T], Generic[_T]):
     def __init__(self, *iterables: Iterable[_T]) -> None: ...