diff --git a/scheme/lib/dns.scm b/scheme/lib/dns.scm
index 1218658..eeab0c4 100644
--- a/scheme/lib/dns.scm
+++ b/scheme/lib/dns.scm
@@ -565,15 +565,26 @@
 	 (values (make-question name type class) start))))))
 
 ;; parses a resourcerecord in a message. returns the rr and the rest of the message.
+
+(define (type-number->type type-number)
+  (if (>= type-number (vector-length the-message-types))
+      'unsupported-message-type
+      (vector-ref the-message-types type-number)))
+
+(define (class-number->class class-number)
+  (if (>= class-number (vector-length the-message-classes))
+      'unsupported-message-class
+      (vector-ref the-message-classes class-number)))
+
 (define (parse-rr start message)
   (call-with-values
    (lambda () (parse-name start message))
    (lambda (name start)
-     (let ((type  (vector-ref the-message-types 
-			      (octet-pair->number (car start) (cadr start)))) 
+     (let ((type  (type-number->type
+                   (octet-pair->number (car start) (cadr start))))
 	   (start (cddr start)))
-       (let ((class (vector-ref the-message-classes 
-				(octet-pair->number (car start) (cadr start))))
+       (let ((class (class-number->class
+                     (octet-pair->number (car start) (cadr start))))
 	     (start (cddr start)))
 	 (let ((ttl (octet-quad->number (car start) (cadr start)
 					(caddr start) (cadddr start)))