diff --git a/Objects/tupleobject.c b/Objects/tupleobject.c
index 0faedb4..d6fb6b4 100644
--- a/Objects/tupleobject.c
+++ b/Objects/tupleobject.c
@@ -133,7 +133,7 @@
 {
 	register PyObject *olditem;
 	register PyObject **p;
-	if (!PyTuple_Check(op)) {
+	if (!PyTuple_Check(op) || op->ob_refcnt != 1) {
 		Py_XDECREF(newitem);
 		PyErr_BadInternalCall();
 		return -1;