diff --git a/src/asynkit/tools.py b/src/asynkit/tools.py
index 06e8dba..f276ffb 100644
--- a/src/asynkit/tools.py
+++ b/src/asynkit/tools.py
@@ -300,8 +300,15 @@ def __lt__(self, other: PriEntry[P, T]) -> bool:
 
 
 class Cancellable(Protocol):
-    def cancel(self, msg: Optional[str] = None) -> Union[bool, None]:
-        ...
+    if sys.version_info >= (3, 9):  # pragma: no cover
+
+        def cancel(self, msg: Optional[str] = None) -> Any:
+            ...
+
+    else:  # pragma: no cover
+
+        def cancel(self) -> Any:
+            ...
 
     def cancelled(self) -> bool:
         ...