diff --git a/Lib/test/test_calendar.py b/Lib/test/test_calendar.py
index 7ade4271b..ca93e99d1 100644
--- a/Lib/test/test_calendar.py
+++ b/Lib/test/test_calendar.py
@@ -457,12 +457,17 @@ class OutputTestCase(unittest.TestCase):
             calendar.TextCalendar().formatmonth(0, 2),
             result_0_02_text
         )
+
     def test_formatmonth_with_invalid_month(self):
         with self.assertRaises(calendar.IllegalMonthError):
             calendar.TextCalendar().formatmonth(2017, 13)
         with self.assertRaises(calendar.IllegalMonthError):
             calendar.TextCalendar().formatmonth(2017, -1)
 
+    def test_illegal_month_error_bases(self):
+        self.assertIsSubclass(calendar.IllegalMonthError, ValueError)
+        self.assertIsSubclass(calendar.IllegalMonthError, IndexError)
+
     def test_formatmonthname_with_year(self):
         self.assertEqual(
             calendar.HTMLCalendar().formatmonthname(2004, 1, withyear=True),