diff --git a/src/snanosm/mealy.py b/src/snanosm/mealy.py
index a282871..6956b7a 100644
--- a/src/snanosm/mealy.py
+++ b/src/snanosm/mealy.py
@@ -50,7 +50,7 @@ class Transition(Generic[TransitionInput]):
 
 
 class Machine(Generic[Input]):
-    def __init__(self, initial_context=None) -> None:
+    def __init__(self, initial_context: object = None) -> None:
         if initial_context is None:
             initial_context = {}
         self.__start_state_hash: Optional[int] = None