diff --git a/networktype.go b/networktype.go
index 9509101..50d96ac 100644
--- a/networktype.go
+++ b/networktype.go
@@ -7,8 +7,12 @@ import (
 )
 
 const (
-	udp = "udp"
-	tcp = "tcp"
+	udp  = "udp"
+	tcp  = "tcp"
+	udp4 = "udp4"
+	udp6 = "udp6"
+	tcp4 = "tcp4"
+	tcp6 = "tcp6"
 )
 
 func supportedNetworkTypes() []NetworkType {
@@ -40,13 +44,13 @@ const (
 func (t NetworkType) String() string {
 	switch t {
 	case NetworkTypeUDP4:
-		return "udp4"
+		return udp4
 	case NetworkTypeUDP6:
-		return "udp6"
+		return udp6
 	case NetworkTypeTCP4:
-		return "tcp4"
+		return tcp4
 	case NetworkTypeTCP6:
-		return "tcp6"
+		return tcp6
 	default:
 		return ErrUnknownType.Error()
 	}
diff --git a/udp_mux_multi_test.go b/udp_mux_multi_test.go
index db69c11..6644963 100644
--- a/udp_mux_multi_test.go
+++ b/udp_mux_multi_test.go
@@ -63,13 +63,13 @@ func TestMultiUDPMux(t *testing.T) {
 	wg.Add(1)
 	go func() {
 		defer wg.Done()
-		testMultiUDPMuxConnections(t, udpMuxMulti, "ufrag2", "udp4")
+		testMultiUDPMuxConnections(t, udpMuxMulti, "ufrag2", udp4)
 	}()
 
 	// skip ipv6 test on i386
 	const ptrSize = 32 << (^uintptr(0) >> 63)
 	if ptrSize != 32 {
-		testMultiUDPMuxConnections(t, udpMuxMulti, "ufrag3", "udp6")
+		testMultiUDPMuxConnections(t, udpMuxMulti, "ufrag3", udp6)
 	}
 
 	wg.Wait()
@@ -87,9 +87,9 @@ func testMultiUDPMuxConnections(t *testing.T, udpMuxMulti *MultiUDPMuxDefault, u
 	for _, addr := range addrs {
 		udpAddr, ok := addr.(*net.UDPAddr)
 		require.True(t, ok)
-		if network == "udp4" && udpAddr.IP.To4() == nil {
+		if network == udp4 && udpAddr.IP.To4() == nil {
 			continue
-		} else if network == "udp6" && udpAddr.IP.To4() != nil {
+		} else if network == udp6 && udpAddr.IP.To4() != nil {
 			continue
 		}
 		c, err := udpMuxMulti.GetConn(ufrag, addr)
@@ -138,13 +138,13 @@ func TestUnspecifiedUDPMux(t *testing.T) {
 	wg.Add(1)
 	go func() {
 		defer wg.Done()
-		testMultiUDPMuxConnections(t, udpMuxMulti, "ufrag2", "udp4")
+		testMultiUDPMuxConnections(t, udpMuxMulti, "ufrag2", udp4)
 	}()
 
 	// skip ipv6 test on i386
 	const ptrSize = 32 << (^uintptr(0) >> 63)
 	if ptrSize != 32 {
-		testMultiUDPMuxConnections(t, udpMuxMulti, "ufrag3", "udp6")
+		testMultiUDPMuxConnections(t, udpMuxMulti, "ufrag3", udp6)
 	}
 
 	wg.Wait()
diff --git a/udp_mux_test.go b/udp_mux_test.go
index 9c90c58..557e453 100644
--- a/udp_mux_test.go
+++ b/udp_mux_test.go
@@ -33,8 +33,8 @@ func TestUDPMux(t *testing.T) {
 		t.Log("IPv6 is not supported on this machine")
 	}
 
-	for network, c := range map[string]net.PacketConn{"udp4": conn4, "udp6": conn6} {
-		if c == nil {
+	for network, c := range map[string]net.PacketConn{udp4: conn4, udp6: conn6} {
+		if udpConn, ok := c.(*net.UDPConn); !ok || udpConn == nil {
 			continue
 		}
 		conn := c
@@ -63,7 +63,7 @@ func TestUDPMux(t *testing.T) {
 
 			// skip ipv6 test on i386
 			const ptrSize = 32 << (^uintptr(0) >> 63)
-			if ptrSize != 32 || network != "udp6" {
+			if ptrSize != 32 || network != udp6 {
 				testMuxConnection(t, udpMux, "ufrag2", network)
 			}