diff --git a/fcore/text-window.sml b/fcore/text-window.sml
index 1dc157e..ec5a2bb 100644
--- a/fcore/text-window.sml
+++ b/fcore/text-window.sml
@@ -187,7 +187,7 @@ struct
             else if cursorIdx > absIdx then
               (* possibly move downwards *)
               getStartLineAfter
-                ( startIdx
+                ( Int.max (startIdx, 0)
                 , rStrHd
                 , oldLine
                 , absIdx