diff --git a/google/auth/_helpers.py b/google/auth/_helpers.py
index 92c6c83..5a6c3a8 100644
--- a/google/auth/_helpers.py
+++ b/google/auth/_helpers.py
@@ -35,7 +35,7 @@
         """Decorator implementation.
 
         Args:
-            method: The method to copy the docstring to.
+            method (Callable): The method to copy the docstring to.
 
         Returns:
             Callable: the same method passed in with an updated docstring.