diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py
index e44802369d..1034825415 100644
--- a/scripts/qapi/schema.py
+++ b/scripts/qapi/schema.py
@@ -989,7 +989,9 @@ def lookup_entity(self, name, typ=None):
         return ent
 
     def lookup_type(self, name):
-        return self.lookup_entity(name, QAPISchemaType)
+        typ = self.lookup_entity(name, QAPISchemaType)
+        assert typ is None or isinstance(typ, QAPISchemaType)
+        return typ
 
     def resolve_type(self, name, info, what):
         typ = self.lookup_type(name)