diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -365,16 +365,45 @@ class WarnTests(BaseTest):
             """Warning with a bad format string for __str__."""
             def __str__(self):
                 return ("A bad formatted string %(err)" %
                         {"err" : "there is no %(err)s"})
 
         with self.assertRaises(ValueError):
             self.module.warn(BadStrWarning())
 
+    def test_warning_classes(self):
+        class MyWarningClass(Warning):
+            pass
+
+        class NonWarningSubclass:
+            pass
+
+        msg_regex = 'category must be a Warning subclass, not (.*)'
+
+        # passing a non-subclass of Warning should raise a TypeError
+        with self.assertRaisesRegex(TypeError, msg_regex):
+            self.module.warn('bad warning category', '')
+
+        with self.assertRaisesRegex(TypeError, msg_regex):
+            self.module.warn('bad warning category', NonWarningSubclass)
+
+        # check that warning instances also raise a TypeError
+        with self.assertRaisesRegex(TypeError, msg_regex):
+            self.module.warn('bad warning category', MyWarningClass())
+
+        with self.assertWarnsRegex(MyWarningClass, 'good warning category'):
+            self.module.warn('good warning category', MyWarningClass)
+
+        with self.assertWarnsRegex(UserWarning, 'good warning category'):
+            self.module.warn('good warning category', None)
+
+        with self.assertWarns(MyWarningClass) as cm:
+            self.module.warn('good warning category', MyWarningClass)
+        self.assertIsInstance(cm.warning, Warning)
 
 class CWarnTests(WarnTests, unittest.TestCase):
     module = c_warnings
 
     # As an early adopter, we sanity check the
     # test.support.import_fresh_module utility function
     def test_accelerated(self):
         self.assertFalse(original_warnings is self.module)
diff --git a/Lib/warnings.py b/Lib/warnings.py
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -157,17 +157,21 @@ def _getcategory(category):
 def warn(message, category=None, stacklevel=1):
     """Issue a warning, or maybe ignore it or raise an exception."""
     # Check if message is already a Warning object
     if isinstance(message, Warning):
         category = message.__class__
     # 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
+    if not (isinstance(category, type) and issubclass(category, Warning)):
+        raise TypeError('category must be a Warning subclass, '
+                        'not {!r}'.format(category))
     # Get context information
     try:
         caller = sys._getframe(stacklevel)
     except ValueError:
         globals = sys.__dict__
         lineno = 1
     else:
         globals = caller.f_globals
diff --git a/Python/_warnings.c b/Python/_warnings.c
--- a/Python/_warnings.c
+++ b/Python/_warnings.c
@@ -614,26 +614,26 @@ get_category(PyObject *message, PyObject
 
     /* Get category. */
     rc = PyObject_IsInstance(message, PyExc_Warning);
     if (rc == -1)
         return NULL;
 
     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 PyObject_IsSubclass */
+    if (rc == -1 || rc == 0) {
+        PyErr_Format(PyExc_TypeError,
+                     "category must be a Warning subclass, not %R",
+                     category);
         return NULL;
     }
 
     return category;
 }
 
 static PyObject *
 do_warn(PyObject *message, PyObject *category, Py_ssize_t stack_level)