diff --git a/collects/web-server/private/xexpr.ss b/collects/web-server/private/xexpr.ss
index 5ce48bbea4..9fd5e15888 100644
--- a/collects/web-server/private/xexpr.ss
+++ b/collects/web-server/private/xexpr.ss
@@ -95,7 +95,11 @@
       l)]))
 
 (define (format-elements l)
-  (map format-xexpr l))
+  (if (list? l)
+      (map format-xexpr l)
+      (mark-error
+       "Expected a list of elements"
+       l)))
 
 (define (format-attributes l)
   (match l