diff --git a/Lib/test/test_str.py b/Lib/test/test_str.py
index 044711c..de988a8 100644
--- a/Lib/test/test_str.py
+++ b/Lib/test/test_str.py
@@ -351,6 +351,10 @@
         self.assertRaises(ValueError, "{:s}".format)
         self.assertRaises(ValueError, "{}".format)
 
+        # issue 6089
+        self.assertRaises(ValueError, "{0[0]x}".format, [None])
+        self.assertRaises(ValueError, "{0[0](10)}".format, [None])
+
         # can't have a replacement on the field name portion
         self.assertRaises(TypeError, '{0[{1}]}'.format, 'abcdefg', 4)
 
diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 968f64b..96c15f0 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1091,6 +1091,10 @@
         self.assertRaises(ValueError, u"{:s}".format)
         self.assertRaises(ValueError, u"{}".format)
 
+        # issue 6089
+        self.assertRaises(ValueError, u"{0[0]x}".format, [None])
+        self.assertRaises(ValueError, u"{0[0](10)}".format, [None])
+
         # can't have a replacement on the field name portion
         self.assertRaises(TypeError, u'{0[{1}]}'.format, u'abcdefg', 4)