diff --git a/dns_monitor.go b/dns_monitor.go
index 59f7f2e..072e01c 100644
--- a/dns_monitor.go
+++ b/dns_monitor.go
@@ -197,11 +197,11 @@ func (m *DnsMonitor) Add(target string, callback DnsMonitorCallback) (*DnsMonito
 		}
 		hostname = parsed.Host
 	} else {
-		// Hostname with optional port passed.
+		// Hostname only passed.
 		hostname = target
-		if h, _, err := net.SplitHostPort(target); err == nil {
-			hostname = h
-		}
+	}
+	if h, _, err := net.SplitHostPort(hostname); err == nil {
+		hostname = h
 	}
 
 	m.mu.Lock()
diff --git a/dns_monitor_test.go b/dns_monitor_test.go
index eedae1c..5844501 100644
--- a/dns_monitor_test.go
+++ b/dns_monitor_test.go
@@ -240,7 +240,7 @@ func TestDnsMonitor(t *testing.T) {
 	rec1 := newDnsMonitorReceiverForTest(t)
 	rec1.Expect(ips1, ips1, nil, nil)
 
-	entry1, err := monitor.Add("https://foo", rec1.OnLookup)
+	entry1, err := monitor.Add("https://foo:12345", rec1.OnLookup)
 	if err != nil {
 		t.Fatal(err)
 	}