diff -r 6952c14a3f75 Lib/_strptime.py
--- a/Lib/_strptime.py	Fri Nov 08 21:08:46 2013 +0100
+++ b/Lib/_strptime.py	Sun Nov 10 13:07:57 2013 +0200
@@ -329,7 +329,7 @@
                                     (bad_directive, format)) from None
             # IndexError only occurs when the format string is "%"
             except IndexError:
-                raise ValueError("stray %% in format '%s'" % format)
+                raise ValueError("stray %% in format '%s'" % format) from None
             _regex_cache[format] = format_regex
     found = format_regex.match(data_string)
     if not found:
diff -r 6952c14a3f75 Lib/test/test_strptime.py
--- a/Lib/test/test_strptime.py	Fri Nov 08 21:08:46 2013 +0100
+++ b/Lib/test/test_strptime.py	Sun Nov 10 13:07:57 2013 +0200
@@ -224,6 +224,11 @@
             _strptime._strptime_time('', '%D')
         self.assertIs(e.exception.__suppress_context__, True)
 
+        # additional check for IndexError branch
+        with self.assertRaises(ValueError) as e:
+            _strptime._strptime_time('19', '%Y %')
+        self.assertIs(e.exception.__suppress_context__, True)
+
     def test_unconverteddata(self):
         # Check ValueError is raised when there is unconverted data
         self.assertRaises(ValueError, _strptime._strptime_time, "10 12", "%m")