diff --git a/scripts/qapi/introspect.py b/scripts/qapi/introspect.py
index 715220afe7..96dfbb4cef 100644
--- a/scripts/qapi/introspect.py
+++ b/scripts/qapi/introspect.py
@@ -126,7 +126,9 @@ def indent(level):
     elif isinstance(obj, bool):
         ret += 'QLIT_QBOOL(%s)' % ('true' if obj else 'false')
     else:
-        assert False                # not implemented
+        raise NotImplementedError(
+            f"type '{type(obj).__name__}' not implemented"
+        )
     if level > 0:
         ret += ','
     return ret