diff --git a/google/auth/credentials.py b/google/auth/credentials.py
index 5ea36a0..bc42546 100644
--- a/google/auth/credentials.py
+++ b/google/auth/credentials.py
@@ -138,7 +138,7 @@
     """Abstract base for credentials supporting ``with_quota_project`` factory"""
 
     def with_quota_project(self, quota_project_id):
-        """Returns a copy of these credentials with a modified quota project
+        """Returns a copy of these credentials with a modified quota project.
 
         Args:
             quota_project_id (str): The project to use for quota and