diff --git a/piclib/built-in.scm b/piclib/built-in.scm
index 39a9509d..35aeef3e 100644
--- a/piclib/built-in.scm
+++ b/piclib/built-in.scm
@@ -42,8 +42,8 @@
                          (cons (r 'lambda) (cons (map car bindings) body)))
                    (cons name (map cadr bindings))))
            (begin
-             (set! bindings (cadr expr))
-             (set! body (cddr expr))
+             (define bindings (cadr expr))
+             (define body (cddr expr))
              (cons (cons (r 'lambda) (cons (map car bindings) body))
                    (map cadr bindings)))))))