diff --git a/zonedb.rkt b/zonedb.rkt
index eb4caa1..50af311 100644
--- a/zonedb.rkt
+++ b/zonedb.rkt
@@ -168,7 +168,7 @@
 (define (filter-by-type rrset type)
   (set-filter (lambda (rr) (eqv? (rr-type rr) type)) rrset))
 
-;; filter-rrs : SetOf<RR> QueryType QueryClass
+;; filter-rrs : SetOf<RR> QueryType QueryClass -> SetOf<RR>
 ;; Returns a set like its argument with RRs not matching the given
 ;; type and class removed.
 (define (filter-rrs rrs qtype qclass)