diff --git a/collects/handin-server/checker.ss b/collects/handin-server/checker.ss
index b51f4adfd0..aaa896522a 100644
--- a/collects/handin-server/checker.ss
+++ b/collects/handin-server/checker.ss
@@ -320,7 +320,9 @@
 (provide add-header-line!)
 (define (add-header-line! line)
   (let ([new (list line)] [cur (thread-cell-ref added-lines)])
-    (if cur (append cur new) (thread-cell-set! added-lines new))))
+    (if cur
+	(set-box! cur (append (unbox cur) new))
+	(thread-cell-set! added-lines (box new)))))
 
 (define ((wrap-evaluator eval) expr)
   (define unknown "unknown")
@@ -477,7 +479,7 @@
                            (prefix-line (user-substs user student-line)))
                          (for-each prefix-line/substs extra-lines)
                          (for-each prefix-line/substs
-                                   (or (thread-cell-ref added-lines) '()))
+				   (unbox (or (thread-cell-ref added-lines) (box '()))))
                          (display submission-text))))
                    (define submission-text
                      (and create-text?
@@ -697,6 +699,15 @@
          (error* "your code failed a test: ~e evaluated to ~e, expecting ~e"
                  (->disp 'expr) (->disp val) (->disp result))))]))
 
+(provide !test/exn)
+(define-syntax (!test/exn stx)
+  (syntax-case stx ()
+     [(_ test-exp)
+      #`(with-handlers ([exn:fail? (lambda (exn) #t)])
+	  ((submission-eval) `test-exp)
+	  (error* "expected exception on test expression: ~v" 
+		  (->disp 'test-exp)))]))
+
 (provide !all-covered)
 (define coverage-checked (make-thread-cell #f))
 (define (!all-covered . proc)