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
@@ -180,16 +180,32 @@ class FilterTests(BaseTest):
             self.module.resetwarnings()
             self.module.filterwarnings("error", "hex*", Warning, "", 0)
             self.assertRaises(UserWarning, self.module.warn, 'hex/oct')
             text = 'nonmatching text'
             self.module.warn(text)
             self.assertEqual(str(w[-1].message), text)
             self.assertTrue(w[-1].category is UserWarning)
 
+    def test_filterwarnings_invalid_category(self):
+        class MyWarningClass(Warning):
+            pass
+
+        class NonWarningSubclass:
+            pass
+
+        msg_regex = 'category must be a Warning subclass, not (.*)'
+
+        with self.assertRaisesRegex(TypeError, msg_regex):
+            self.module.filterwarnings("always", category=MyWarningClass())
+
+        with self.assertRaisesRegex(TypeError, msg_regex):
+            self.module.filterwarnings("always", category=NonWarningSubclass)
+
+
 class CFilterTests(FilterTests, unittest.TestCase):
     module = c_warnings
 
 class PyFilterTests(FilterTests, unittest.TestCase):
     module = py_warnings
 
 
 class WarnTests(BaseTest):
diff --git a/Lib/warnings.py b/Lib/warnings.py
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -36,21 +36,22 @@ def filterwarnings(action, message="", c
     'module' -- a regex that the module name must match
     'lineno' -- an integer line number, 0 matches all warnings
     'append' -- if true, append to the list of filters
     """
     import re
     assert action in ("error", "ignore", "always", "default", "module",
                       "once"), "invalid action: %r" % (action,)
     assert isinstance(message, str), "message must be a string"
-    assert isinstance(category, type), "category must be a class"
-    assert issubclass(category, Warning), "category must be a Warning subclass"
     assert isinstance(module, str), "module must be a string"
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
+    if not (isinstance(category, type) and issubclass(category, Warning)):
+        raise TypeError('category must be a Warning subclass, '
+                        'not {!r}'.format(category))
     item = (action, re.compile(message, re.I), category,
             re.compile(module), lineno)
     if append:
         filters.append(item)
     else:
         filters.insert(0, item)
 
 def simplefilter(action, category=Warning, lineno=0, append=False):