diff --git a/pkgs/racket-test-extra/tests/ffi/union.rkt b/pkgs/racket-test-extra/tests/ffi/union.rkt
new file mode 100644
index 0000000000..9029d3729f
--- /dev/null
+++ b/pkgs/racket-test-extra/tests/ffi/union.rkt
@@ -0,0 +1,31 @@
+#lang racket/base
+
+;; Tests for FFI unions
+
+(require ffi/unsafe
+         rackunit)
+
+(define union-type (_union (_list-struct _int _int)))
+(define union-type-2 (_union (_list-struct _int _int)
+                             (_list-struct _double _double)))
+
+(define val (cast (list 1 2) (_list-struct _int _int) union-type))
+(define val-2 (cast (list 1.2 2.2) (_list-struct _double _double) union-type-2))
+
+(check-equal? (car (union-ref val 0)) 1)
+(check-equal? (car (union-ref val-2 1)) 1.2)
+
+(union-set! val-2 0 (list 5 4))
+(check-equal? (car (union-ref val-2 0)) 5)
+
+(check-exn #rx"expected: list of c types" (λ () (_union 3)))
+(check-exn #rx"expected: list of c types" (λ () (_union _int 4)))
+(check-not-exn (λ () (_union _int _int)))
+
+(check-exn #rx"too large" (λ () (union-ref val 1)))
+(check-exn #rx"nonnegative-integer" (λ () (union-ref val -1)))
+(check-exn #rx"nonnegative-integer" (λ () (union-ref val "foo")))
+(check-exn #rx"too large" (λ () (union-ref val-2 2)))
+(check-exn #rx"too large" (λ () (union-set! val 1 (list 1 2))))
+(check-exn #rx"nonnegative-integer" (λ () (union-set! val -1 (list 1 2))))
+(check-exn #rx"nonnegative-integer" (λ () (union-set! val "foo" (list 1 2))))
diff --git a/racket/collects/ffi/unsafe.rkt b/racket/collects/ffi/unsafe.rkt
index 6829304bc8..18260f5f3e 100644
--- a/racket/collects/ffi/unsafe.rkt
+++ b/racket/collects/ffi/unsafe.rkt
@@ -1233,6 +1233,8 @@
          (protect-out union-ref union-set!))
 
 (define (_union t . ts)
+  (unless (and (ctype? t) (andmap ctype? ts))
+    (raise-argument-error '_union "list of c types" (cons t ts)))
   (let ([ts (cons t ts)])
     (make-ctype (apply make-union-type ts)
                 (lambda (v) (union-ptr v))
@@ -1240,8 +1242,26 @@
 
 (define-struct union (ptr types))
 (define (union-ref u i)
+  (unless (union? u)
+    (raise-argument-error 'union-ref "union value" 0 u i))
+  (unless (exact-nonnegative-integer? i)
+    (raise-argument-error 'union-ref "exact-nonnegative-integer?" 1 u i))
+  (unless (< i (length (union-types u)))
+    (raise-arguments-error 'union-ref
+                           "index too large for union"
+                           "index"
+                           i))
   (ptr-ref (union-ptr u) (list-ref (union-types u) i)))
 (define (union-set! u i v)
+  (unless (union? u)
+    (raise-argument-error 'union-ref "union value" 0 u i))
+  (unless (exact-nonnegative-integer? i)
+    (raise-argument-error 'union-ref "exact-nonnegative-integer?" 1 u i))
+  (unless (< i (length (union-types u)))
+    (raise-arguments-error 'union-ref
+                           "index too large for union"
+                           "index"
+                           i))
   (ptr-set! (union-ptr u) (list-ref (union-types u) i) v))
 
 ;; ----------------------------------------------------------------------------