diff --git a/scripts/qapi/error.py b/scripts/qapi/error.py
index d179a3bd0c..d0bc7af6e7 100644
--- a/scripts/qapi/error.py
+++ b/scripts/qapi/error.py
@@ -25,6 +25,7 @@ class QAPISourceError(QAPIError):
         self.col = col
 
     def __str__(self):
+        assert self.info is not None
         loc = str(self.info)
         if self.col is not None:
             assert self.info.line is not None