diff --git a/src/KitCopy.ml b/src/KitCopy.ml
index 85f219cc0b47c845946da5921efe511671ec8739..6a1e6654c24a8d6a0dc8455f2311e5711bcb2b87 100644
--- a/src/KitCopy.ml
+++ b/src/KitCopy.ml
@@ -27,6 +27,6 @@ let extend x env =
   x', Atom.Map.add x x' env
 
 class ['self] map = object (_ : 'self)
-  method private extend = extend
-  method private visit_'fn = lookup
+  method private extend x env = extend x env
+  method private visit_'fn env x = lookup env x
 end