diff --git a/fcore/persistent-vector.sml b/fcore/persistent-vector.sml
index 2b377d6..5da1c91 100644
--- a/fcore/persistent-vector.sml
+++ b/fcore/persistent-vector.sml
@@ -814,7 +814,7 @@ struct
           val left = splitLeft (start, tree)
           val right = splitRight (finish, tree)
 
-          val left = append (start, finish, tree)
+          val left = append (start, finish, left)
 
           val rightStartRelative = getStartIdx right
           val rightStartAbsolute = rightStartRelative + finish