diff -r 3aec82018a18 Lib/test/test_exceptions.py
--- a/Lib/test/test_exceptions.py	Tue Mar 22 09:11:39 2011 -0700
+++ b/Lib/test/test_exceptions.py	Tue Mar 22 23:38:53 2011 +0100
@@ -708,6 +708,15 @@
             self.fail("RuntimeError not raised")
         self.assertEqual(wr(), None)
 
+    def test_new_returns_invalid_instance(self):
+        # See issue #11627.
+        class MyException(Exception):
+            def __new__(cls, *args):
+                return object()
+
+        with self.assertRaises(TypeError):
+            raise MyException
+
 def test_main():
     run_unittest(ExceptionTests)
 
diff -r 3aec82018a18 Python/ceval.c
--- a/Python/ceval.c	Tue Mar 22 09:11:39 2011 -0700
+++ b/Python/ceval.c	Tue Mar 22 23:38:53 2011 +0100
@@ -3249,6 +3249,13 @@
         value = PyObject_CallObject(exc, NULL);
         if (value == NULL)
             goto raise_error;
+        else if (!PyExceptionInstance_Check(value)) {
+            PyErr_Format(PyExc_TypeError,
+                         "calling %R should have returned an instance of "
+                         "BaseException, not %R",
+                         type, Py_TYPE(value));
+            goto raise_error;
+        }
     }
     else if (PyExceptionInstance_Check(exc)) {
         value = exc;