diff -r 9f464eff218a Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py	Wed Oct 31 12:03:48 2012 +0200
+++ b/Lib/test/test_warnings.py	Thu Nov 01 13:22:25 2012 +0000
@@ -359,6 +359,27 @@
         with self.assertRaises(ValueError):
             self.module.warn(BadStrWarning())
 
+    def test_warning_classes(self):
+        class MyWarningClass(Warning):
+            pass
+        
+        # passing a non-subclass of Warning should raise a ValueError
+        with self.assertRaises(ValueError):
+            self.module.warn('bad warning category', '')
+        # check that warning instances also raise a ValueError
+        with self.assertRaises(ValueError):
+            self.module.warn('bad warning category', MyWarningClass())
+
+        with warnings_state(self.module):
+            with original_warnings.catch_warnings(record=True,
+                    module=self.module) as w:
+            
+                self.module.warn('good warning category', MyWarningClass)
+                self.assertEqual(str(w[-1].message), 'good warning category')
+
+                self.module.warn('good warning category', None)
+                self.assertEqual(str(w[-1].message), 'good warning category')
+              
 
 class CWarnTests(BaseTest, WarnTests):
     module = c_warnings
diff -r 9f464eff218a Lib/warnings.py
--- a/Lib/warnings.py	Wed Oct 31 12:03:48 2012 +0200
+++ b/Lib/warnings.py	Thu Nov 01 13:22:25 2012 +0000
@@ -165,7 +165,14 @@
     # Check category argument
     if category is None:
         category = UserWarning
-    assert issubclass(category, Warning)
+    # user-error means that category could be either a class
+    # or some invalid object instance
+    try:
+        if not issubclass(category, Warning):
+            raise TypeError('')
+    except TypeError:
+        raise ValueError('category must be a subclass of {!s}.'
+                         'Got {!r}'.format(Warning, category)) from None
     # Get context information
     try:
         caller = sys._getframe(stacklevel)
diff -r 9f464eff218a Python/_warnings.c
--- a/Python/_warnings.c	Wed Oct 31 12:03:48 2012 +0200
+++ b/Python/_warnings.c	Thu Nov 01 13:22:25 2012 +0000
@@ -586,16 +586,16 @@
 
     if (rc == 1)
         category = (PyObject*)message->ob_type;
-    else if (category == NULL)
+    else if (category == NULL || category == Py_None)
         category = PyExc_UserWarning;
 
     /* Validate category. */
     rc = PyObject_IsSubclass(category, PyExc_Warning);
-    if (rc == -1)
-        return NULL;
-    if (rc == 0) {
-        PyErr_SetString(PyExc_ValueError,
-                        "category is not a subclass of Warning");
+    /* either not a subclass OR an error from IsSubclass */
+    if (rc == -1 || rc == 0) {
+        PyErr_Format(PyExc_ValueError,
+                     "category must be a subclass of %S. Got %R.",
+                     PyExc_Warning, category);
         return NULL;
     }