diff --git a/collects/ffi/unsafe/objc.rkt b/collects/ffi/unsafe/objc.rkt
index a371abc34c..1166f80248 100644
--- a/collects/ffi/unsafe/objc.rkt
+++ b/collects/ffi/unsafe/objc.rkt
@@ -372,7 +372,9 @@
 (define (free-fields obj names)
   (for-each (lambda (name)
               (let-values ([(ivar p) (object_getInstanceVariable obj name)])
-                (when p (free-immobile-cell p))))
+                (when p
+                  (object_setInstanceVariable obj name #f)
+                  (free-immobile-cell p))))
             names))
 
 ;; ----------------------------------------