diff --git a/mercurial/cext/bdiff.c b/mercurial/cext/bdiff.c
--- a/mercurial/cext/bdiff.c
+++ b/mercurial/cext/bdiff.c
@@ -261,7 +261,8 @@
                          void *priv)
 {
 	PyObject *rl = (PyObject *)priv;
-	PyObject *m = Py_BuildValue("llll", a1, a2, b1, b2);
+	Py_BUILD_ASSERT(sizeof(a1) == sizeof(PY_LONG_LONG));
+	PyObject *m = Py_BuildValue("LLLL", a1, a2, b1, b2);
 	if (!m)
 		return -1;
 	if (PyList_Append(rl, m) != 0) {