diff --git a/src/util/Backtrack_stack.ml b/src/util/Backtrack_stack.ml
index b4d955da..87e8b702 100644
--- a/src/util/Backtrack_stack.ml
+++ b/src/util/Backtrack_stack.ml
@@ -8,7 +8,7 @@ type 'a t = {
 
 let create() : _ t = {vec=Vec.create(); lvls=Vec.create()}
 
-let[@inline] n_levels self : int = Vec.size self.vec
+let[@inline] n_levels self : int = Vec.size self.lvls
 
 let[@inline] push self x : unit =
   Vec.push self.vec x
@@ -21,9 +21,10 @@ let[@inline] push_if_nonzero_level self x : unit =
 let[@inline] push_level (self:_ t) : unit =
   Vec.push self.lvls (Vec.size self.vec)
 
-
 let pop_levels (self:_ t) (n:int) ~f : unit =
-  if n > n_levels self then invalid_arg "Backtrack_stack.pop_levels";
+  if n > n_levels self then (
+    Error.errorf "Backtrack_stack.pop_levels %d (size: %d)" n (n_levels self);
+  );
   if n > 0 then (
     let new_lvl = n_levels self - n in
     let i = Vec.get self.lvls new_lvl in