diff --git a/pkgs/racket-test-core/tests/racket/will.rktl b/pkgs/racket-test-core/tests/racket/will.rktl
index b7d5c6ebfb..9017e7ff56 100644
--- a/pkgs/racket-test-core/tests/racket/will.rktl
+++ b/pkgs/racket-test-core/tests/racket/will.rktl
@@ -127,7 +127,9 @@
     (collect-garbage)
     (test #t andmap (lambda (v)
                       (v . >= . 100000))
-          (map current-memory-use c))))
+          (map current-memory-use c))
+    ;; Make sure boxes are retained:
+    (test #t andmap custodian-box? b)))
 
 (let ()
   (define c1 (make-custodian (current-custodian)))