diff --git a/dns.go b/dns.go
index fab5fe1..43043f0 100644
--- a/dns.go
+++ b/dns.go
@@ -243,10 +243,13 @@ func (m *DNSMetrics) Collect() {
 
 	for ix, server := range dns_servers {
 		if server.role == "master" {
-			soa := results[ix].serial
-			masters = append(masters, soa)
-			if soa > latest_soa {
-				latest_soa = soa
+			// check that a result was actually received by the master
+			if results[ix] != nil {
+				soa := results[ix].serial
+				masters = append(masters, soa)
+				if soa > latest_soa {
+					latest_soa = soa
+				}
 			}
 		}
 	}