diff --git a/scripts/qapi/visit.py b/scripts/qapi/visit.py
index 678109dfb5..d5d7a1031f 100644
--- a/scripts/qapi/visit.py
+++ b/scripts/qapi/visit.py
@@ -232,6 +232,7 @@ def gen_visit_alternate(name, variants):
     case QTYPE_NONE:
         abort();
     default:
+        assert(visit_is_input(v));
         error_setg(&err, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
                    "%(name)s");
         /* Avoid passing invalid *obj to qapi_free_%(c_name)s() */