diff --git a/Objects/setobject.c b/Objects/setobject.c
index 29a9184..d39f265 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -1214,7 +1214,7 @@
 	return (PyObject *)so;
 }
 
-int
+static int
 set_difference_update_internal(PySetObject *so, PyObject *other)
 {
 	if ((PyObject *)so == other)