diff --git a/Lib/inspect.py b/Lib/inspect.py
index f62167b..c358a5c 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -173,7 +173,7 @@
             return object.__file__
         raise TypeError, 'arg is a built-in module'
     if isclass(object):
-        object = sys.modules[object.__module__]
+        object = sys.modules.get(object.__module__)
         if hasattr(object, '__file__'):
             return object.__file__
         raise TypeError, 'arg is a built-in class'