diff --git a/src/util-bst-lset.scm b/src/util-bst-lset.scm
index 921a78f..b8d72ff 100644
--- a/src/util-bst-lset.scm
+++ b/src/util-bst-lset.scm
@@ -60,6 +60,8 @@
     (bst-EQ? ls1)))
 
  (define (lset-subtract ls1 ls2)
-   (foldl lset-remove ls1 (lset->list ls2) #t))
+   (foldl (lambda (ls e)
+	    (lset-remove ls e #t))
+	  ls1 (lset->list ls2)))
 
  )