diff --git a/src/lightning/pytorch/core/module.py b/src/lightning/pytorch/core/module.py
index 7df0cb7757f81..836356cbeb79a 100644
--- a/src/lightning/pytorch/core/module.py
+++ b/src/lightning/pytorch/core/module.py
@@ -381,7 +381,7 @@ def log(
         logger: Optional[bool] = None,
         on_step: Optional[bool] = None,
         on_epoch: Optional[bool] = None,
-        reduce_fx: Union[str, Callable] = "mean",
+        reduce_fx: Union[str, Callable[[Any], Any]] = "mean",
         enable_graph: bool = False,
         sync_dist: bool = False,
         sync_dist_group: Optional[Any] = None,
@@ -546,7 +546,7 @@ def log_dict(
         logger: Optional[bool] = None,
         on_step: Optional[bool] = None,
         on_epoch: Optional[bool] = None,
-        reduce_fx: Union[str, Callable] = "mean",
+        reduce_fx: Union[str, Callable[[Any], Any]] = "mean",
         enable_graph: bool = False,
         sync_dist: bool = False,
         sync_dist_group: Optional[Any] = None,