diff -r 2688655e431a Lib/sqlite3/test/factory.py
--- a/Lib/sqlite3/test/factory.py	Thu May 21 14:19:20 2015 +0300
+++ b/Lib/sqlite3/test/factory.py	Thu May 21 18:19:55 2015 +0300
@@ -180,6 +180,14 @@ class RowFactoryTests(unittest.TestCase)
         self.assertEqual(list(reversed(row)), list(reversed(as_tuple)))
         self.assertIsInstance(row, Sequence)
 
+    def CheckFakeCursorClass(self):
+        # Issue #24257: Incorrect use of PyObject_IsInstance() caused
+        # segmentation fault.
+        class FakeCursor(str):
+            __class__ = sqlite.Cursor
+        cur = self.con.cursor(factory=FakeCursor)
+        self.assertRaises(TypeError, sqlite.Row, cur, ())
+
     def tearDown(self):
         self.con.close()
 
diff -r 2688655e431a Lib/test/test_types.py
--- a/Lib/test/test_types.py	Thu May 21 14:19:20 2015 +0300
+++ b/Lib/test/test_types.py	Thu May 21 18:19:55 2015 +0300
@@ -1172,6 +1172,22 @@ class SimpleNamespaceTests(unittest.Test
 
             self.assertEqual(ns, ns_roundtrip, pname)
 
+    def test_fake_namespace_compare(self):
+        # Issue #24257: Incorrect use of PyObject_IsInstance() caused
+        # SystemError.
+        class FakeSimpleNamespace(str):
+            __class__ = types.SimpleNamespace
+        self.assertFalse(types.SimpleNamespace() == FakeSimpleNamespace())
+        self.assertTrue(types.SimpleNamespace() != FakeSimpleNamespace())
+        with self.assertRaises(TypeError):
+            types.SimpleNamespace() < FakeSimpleNamespace()
+        with self.assertRaises(TypeError):
+            types.SimpleNamespace() <= FakeSimpleNamespace()
+        with self.assertRaises(TypeError):
+            types.SimpleNamespace() > FakeSimpleNamespace()
+        with self.assertRaises(TypeError):
+            types.SimpleNamespace() >= FakeSimpleNamespace()
+
 
 class CoroutineTests(unittest.TestCase):
     def test_wrong_args(self):
diff -r 2688655e431a Modules/_sqlite/row.c
--- a/Modules/_sqlite/row.c	Thu May 21 14:19:20 2015 +0300
+++ b/Modules/_sqlite/row.c	Thu May 21 18:19:55 2015 +0300
@@ -46,7 +46,7 @@ pysqlite_row_new(PyTypeObject *type, PyO
     if (!PyArg_ParseTuple(args, "OO", &cursor, &data))
         return NULL;
 
-    if (!PyObject_IsInstance((PyObject*)cursor, (PyObject*)&pysqlite_CursorType)) {
+    if (!PyObject_TypeCheck((PyObject*)cursor, &pysqlite_CursorType)) {
         PyErr_SetString(PyExc_TypeError, "instance of cursor required for first argument");
         return NULL;
     }
diff -r 2688655e431a Objects/genobject.c
--- a/Objects/genobject.c	Thu May 21 14:19:20 2015 +0300
+++ b/Objects/genobject.c	Thu May 21 18:19:55 2015 +0300
@@ -442,8 +442,7 @@ int
         PyErr_Fetch(&et, &ev, &tb);
         if (ev) {
             /* exception will usually be normalised already */
-            if (Py_TYPE(ev) == (PyTypeObject *) et
-                || PyObject_IsInstance(ev, PyExc_StopIteration)) {
+            if (PyObject_TypeCheck(ev, (PyTypeObject *) et)) {
                 value = ((PyStopIterationObject *)ev)->value;
                 Py_INCREF(value);
                 Py_DECREF(ev);
@@ -453,7 +452,7 @@ int
             } else {
                 /* normalisation required */
                 PyErr_NormalizeException(&et, &ev, &tb);
-                if (!PyObject_IsInstance(ev, PyExc_StopIteration)) {
+                if (!PyObject_TypeCheck(ev, (PyTypeObject *)PyExc_StopIteration)) {
                     PyErr_Restore(et, ev, tb);
                     return -1;
                 }
diff -r 2688655e431a Objects/namespaceobject.c
--- a/Objects/namespaceobject.c	Thu May 21 14:19:20 2015 +0300
+++ b/Objects/namespaceobject.c	Thu May 21 18:19:55 2015 +0300
@@ -164,12 +164,11 @@ namespace_clear(_PyNamespaceObject *ns)
 static PyObject *
 namespace_richcompare(PyObject *self, PyObject *other, int op)
 {
-    if (PyObject_IsInstance(self, (PyObject *)&_PyNamespace_Type) &&
-            PyObject_IsInstance(other, (PyObject *)&_PyNamespace_Type))
+    if (PyObject_TypeCheck(self, &_PyNamespace_Type) &&
+        PyObject_TypeCheck(other, &_PyNamespace_Type))
         return PyObject_RichCompare(((_PyNamespaceObject *)self)->ns_dict,
                                    ((_PyNamespaceObject *)other)->ns_dict, op);
-    Py_INCREF(Py_NotImplemented);
-    return Py_NotImplemented;
+    Py_RETURN_NOTIMPLEMENTED;
 }