diff --git a/src/util/Backtrack_stack.ml b/src/util/Backtrack_stack.ml
index 87e8b702..a23056f1 100644
--- a/src/util/Backtrack_stack.ml
+++ b/src/util/Backtrack_stack.ml
@@ -32,5 +32,5 @@ let pop_levels (self:_ t) (n:int) ~f : unit =
       let x = Vec.pop self.vec in
       f x
     done;
-    Vec.shrink self.lvls i
+    Vec.shrink self.lvls new_lvl
   )