diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index bafb501fcf..4d602d7f0a 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -851,7 +851,7 @@ class Popen(object):
 
     def poll(self):
         """Check if child process has terminated. Set and return returncode
-        attribute."""
+        attribute. If the child process hasn't terminated yet, return None."""
         return self._internal_poll()