diff --git a/doc/Changelog b/doc/Changelog
index 71e56f975..b1a1b7557 100644
--- a/doc/Changelog
+++ b/doc/Changelog
@@ -1,5 +1,7 @@
 14 July 2010: Wouter
 	- Donation text added to README.
+	- Fix integer underflow in prefetch ttl creation from cache.  This
+	  fixes a potential negative prefetch ttl.
 
 12 July 2010: Wouter
 	- Changed the defaults for num-queries-per-thread/outgoing-range.
diff --git a/services/cache/dns.c b/services/cache/dns.c
index 81e2f74e8..786ed6ae7 100644
--- a/services/cache/dns.c
+++ b/services/cache/dns.c
@@ -430,9 +430,9 @@ tomsg(struct module_env* env, struct msgreply_entry* e, struct reply_info* r,
 	msg->rep->flags = r->flags;
 	msg->rep->qdcount = r->qdcount;
 	msg->rep->ttl = r->ttl - now;
-	if(r->prefetch_ttl - now > 0)
+	if(r->prefetch_ttl > now)
 		msg->rep->prefetch_ttl = r->prefetch_ttl - now;
-	else	msg->rep->prefetch_ttl = PREFETCH_TTL_CALC(r->prefetch_ttl);
+	else	msg->rep->prefetch_ttl = PREFETCH_TTL_CALC(msg->rep->ttl);
 	msg->rep->security = r->security;
 	msg->rep->an_numrrsets = r->an_numrrsets;
 	msg->rep->ns_numrrsets = r->ns_numrrsets;