diff --git a/codec.rkt b/codec.rkt
index 815a364..e846580 100644
--- a/codec.rkt
+++ b/codec.rkt
@@ -216,7 +216,7 @@
        (tc :: (t:named-bit 'not-truncated 'truncated))
        (rd :: (t:named-bit 'no-recursion-desired 'recursion-desired))
        (ra :: (t:named-bit 'no-recursion-available 'recursion-available))
-       (= 0 :: bits 3)
+       (:: bits 3) ;; previously, had to be zero; now, Z, AD and CD respectively. See RFC 2535.
        (rcode :: bits 4)
 
        (qdcount :: bits 16)