diff --git a/py/modbuiltins.c b/py/modbuiltins.c
index 3355da259d..2a5b7a19cf 100644
--- a/py/modbuiltins.c
+++ b/py/modbuiltins.c
@@ -141,7 +141,7 @@ MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_any_obj, mp_builtin_any);
 
 STATIC mp_obj_t mp_builtin_bin(mp_obj_t o_in) {
     mp_obj_t args[] = { MP_OBJ_NEW_QSTR(MP_QSTR__brace_open__colon__hash_b_brace_close_), o_in };
-    return mp_obj_str_format(MP_ARRAY_SIZE(args), args, MP_OBJ_NULL);
+    return mp_obj_str_format(MP_ARRAY_SIZE(args), args, NULL);
 }
 MP_DEFINE_CONST_FUN_OBJ_1(mp_builtin_bin_obj, mp_builtin_bin);