diff --git a/examples/inverter.py b/examples/inverter.py
index a84002f..e5a29e1 100644
--- a/examples/inverter.py
+++ b/examples/inverter.py
@@ -6,7 +6,7 @@ class Context(TypedDict):
     result: str
     n_chars: int
 
-def add_to_result(context, c):
+def add_to_result(context: Context, c: str) -> None:
     context["result"] += c
     context["n_chars"] += 1