diff --git a/collects/drscheme/private/unit.ss b/collects/drscheme/private/unit.ss
index 4c5d37a8f5..961a0bc35f 100644
--- a/collects/drscheme/private/unit.ss
+++ b/collects/drscheme/private/unit.ss
@@ -1304,6 +1304,7 @@ module browser threading seems wrong.
                                  [lst lst])
                         (cond
                           [(zero? n) null]
+                          [(null? lst) null]
                           [else (cons (car lst) (loop (- n 1) (cdr lst)))])))]
                    [find-exp-diff
                     (λ (p1 p2)