diff --git a/Objects/setobject.c b/Objects/setobject.c
index 8a5f7cf..c8db7ce 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -2198,6 +2198,10 @@
 int
 PySet_Add(PyObject *set, PyObject *key)
 {
+	if (!PyType_IsSubtype(Py_TYPE(set), &PySet_Type)) {
+		PyErr_BadInternalCall();
+		return -1;
+	}
 	return set_add_key((PySetObject *)set, key);
 }
 
@@ -2341,6 +2345,7 @@
 	f = PyFrozenSet_New(dup);
 	assert(PySet_Size(f) == 3);
 	assert(PyFrozenSet_CheckExact(f));
+	assertRaises(PySet_Add(f, elem) == -1, PyExc_SystemError);
 	assertRaises(PySet_Discard(f, elem) == -1, PyExc_SystemError);
 	assertRaises(PySet_Pop(f) == NULL, PyExc_SystemError);
 	Py_DECREF(f);