diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 500dfc9..c5d20c9 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -3031,17 +3031,21 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
         return -1;
     }
     if (!PyType_Check(value)) {
-        PyErr_Format(PyExc_TypeError,
-          "__class__ must be set to new-style class, not '%s' object",
+        PyErr_Format(PyExc_TypeError, "__class__ must be set to a class defined "
+                                      "by a class statement, not '%s' object",
           Py_TYPE(value)->tp_name);
         return -1;
     }
     newto = (PyTypeObject *)value;
-    if (!(newto->tp_flags & Py_TPFLAGS_HEAPTYPE) ||
-        !(oldto->tp_flags & Py_TPFLAGS_HEAPTYPE))
-    {
+    if(!(newto->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
         PyErr_Format(PyExc_TypeError,
-                     "__class__ assignment: only for heap types");
+          "class__ must be set to a class defined by a class statement, not '%s'",
+          newto->tp_name);
+        return -1;
+    }
+    if(!(oldto->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
+        PyErr_SetString(PyExc_TypeError, "__class__ assignment: only for instances "
+                                         "of classes defined by class statements");
         return -1;
     }
     if (compatible_for_assignment(newto, oldto, "__class__")) {