diff --git a/src/util-bst-lset.scm b/src/util-bst-lset.scm
index 0a255d1..921a78f 100644
--- a/src/util-bst-lset.scm
+++ b/src/util-bst-lset.scm
@@ -60,6 +60,6 @@
     (bst-EQ? ls1)))
 
  (define (lset-subtract ls1 ls2)
-   (foldl lset-remove ls1 (lset->list ls2)))
+   (foldl lset-remove ls1 (lset->list ls2) #t))
 
  )