diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py
index 1034825..0ef9b33 100644
--- a/scripts/qapi/schema.py
+++ b/scripts/qapi/schema.py
@@ -996,6 +996,7 @@
     def resolve_type(self, name, info, what):
         typ = self.lookup_type(name)
         if not typ:
+            assert info and what  # built-in types must not fail lookup
             if callable(what):
                 what = what(info)
             raise QAPISemError(