diff --git a/Lib/test/test_unittest.py b/Lib/test/test_unittest.py
index 0812184..ac52e72 100644
--- a/Lib/test/test_unittest.py
+++ b/Lib/test/test_unittest.py
@@ -1580,6 +1580,19 @@
             pass
         else:
             self.fail("Failed to raise TypeError")
+
+    def test_addTest__noncallable(self):
+        suite = unittest.TestSuite()
+        self.assertRaises(TypeError, suite.addTest, 5)
+
+    def test_addTest__casesuiteclass(self):
+        suite = unittest.TestSuite()
+        self.assertRaises(TypeError, suite.addTest, Test_TestSuite)
+        self.assertRaises(TypeError, suite.addTest, unittest.TestSuite)
+
+    def test_addTests__string(self):
+        suite = unittest.TestSuite()
+        self.assertRaises(TypeError, suite.addTests, "foo")
             
         
 class Test_FunctionTestCase(TestCase):
diff --git a/Lib/unittest.py b/Lib/unittest.py
index 0d69f52..483f006 100644
--- a/Lib/unittest.py
+++ b/Lib/unittest.py
@@ -428,9 +428,18 @@
         return cases
 
     def addTest(self, test):
+        # sanity checks
+        if not callable(test):
+            raise TypeError("the test to add must be callable")
+        if (isinstance(test, (type, types.ClassType)) and
+            issubclass(test, (TestCase, TestSuite))):
+            raise TypeError("TestCases and TestSuites must be instantiated "
+                            "before passing them to addTest()")
         self._tests.append(test)
 
     def addTests(self, tests):
+        if isinstance(tests, basestring):
+            raise TypeError("tests must be an iterable of tests, not a string")
         for test in tests:
             self.addTest(test)