diff --git a/src/pythonjsonlogger/jsonlogger.py b/src/pythonjsonlogger/jsonlogger.py
index 519a64d..109148a 100644
--- a/src/pythonjsonlogger/jsonlogger.py
+++ b/src/pythonjsonlogger/jsonlogger.py
@@ -120,7 +120,7 @@ def __init__(
         reserved_attrs: Tuple[str, ...] = RESERVED_ATTRS,
         timestamp: Union[bool, str] = False,
         **kwargs: Any
-    ):
+    ) -> None:
         """
         :param json_default: a function for encoding non-standard objects
             as outlined in https://docs.python.org/3/library/json.html