diff -r 1309fee48908 Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py	Mon Oct 14 20:50:32 2013 +0200
+++ b/Lib/test/test_warnings.py	Tue Oct 15 16:57:29 2013 +0800
@@ -359,6 +359,20 @@
         with self.assertRaises(ValueError):
             self.module.warn(BadStrWarning())
 
+    def test_simplefilter_invalid_category(self):
+        with original_warnings.catch_warnings(record=True,
+                module=self.module) as w:
+            invalid_category = 'spam'
+            with self.assertRaises(AssertionError) as cm:
+                self.module.simplefilter("once", invalid_category)
+            self.assertEqual(str(cm.exception), "category must be a class")
+            class Spam: pass
+            invalid_category = Spam
+            with self.assertRaises(AssertionError) as cm:
+                self.module.simplefilter("once", invalid_category)
+            self.assertEqual(str(cm.exception),
+                    "category must be a Warning subclass")
+
 
 class CWarnTests(WarnTests, unittest.TestCase):
     module = c_warnings
diff -r 1309fee48908 Lib/warnings.py
--- a/Lib/warnings.py	Mon Oct 14 20:50:32 2013 +0200
+++ b/Lib/warnings.py	Tue Oct 15 16:57:29 2013 +0800
@@ -70,6 +70,9 @@
                       "once"), "invalid action: %r" % (action,)
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
+    assert isinstance(category, type), "category must be a class"
+    assert issubclass(category, Warning), \
+        "category must be a Warning subclass"
     item = (action, None, category, None, lineno)
     if append:
         filters.append(item)