diff --git a/src/util/ui_message.h b/src/util/ui_message.h
index 62756df777e..a9becebbf48 100644
--- a/src/util/ui_message.h
+++ b/src/util/ui_message.h
@@ -52,6 +52,7 @@ class ui_message_handlert : public message_handlert
 
   json_stream_arrayt &get_json_stream() override
   {
+    PRECONDITION(json_stream!=nullptr);
     return *json_stream;
   }