diff --git a/collects/racket/set.rkt b/collects/racket/set.rkt
index 4da3d4b8fa..0294a33570 100644
--- a/collects/racket/set.rkt
+++ b/collects/racket/set.rkt
@@ -111,6 +111,7 @@
 
 (define set-union
   (case-lambda
+   [() (set)]
    [(set) 
     (unless (set? set) (raise-type-error 'set-union "set" 0 set))
     set]
diff --git a/collects/tests/racket/set.rktl b/collects/tests/racket/set.rktl
index bb91761c34..e8bd33675a 100644
--- a/collects/tests/racket/set.rktl
+++ b/collects/tests/racket/set.rktl
@@ -17,6 +17,8 @@
 (test #t set-empty? (seteqv))
 (test #t set? (seteqv 1 2 3))
 (test #f set-empty? (seteqv 1 2 3))
+(test #t set? (set-union))
+(test #t set-empty? (set-union))
 
 (test #f set-eq? (set 1 2 3))
 (test #f set-eqv? (set 1 2 3))