diff --git a/tcpuid_linux.go b/tcpuid_linux.go
index 7aaa718..112ac9f 100644
--- a/tcpuid_linux.go
+++ b/tcpuid_linux.go
@@ -44,6 +44,14 @@ func TCPClientUIDSupported() bool {
 // TCPClientUID obtains UID of client process that created
 // TCP connection over the loopback interface
 func TCPClientUID(client, server *net.TCPAddr) (int, error) {
+	// Obtain protocol family. Check for mismatch.
+	clientIs4 := client.IP.To4() != nil
+	serverIs4 := server.IP.To4() != nil
+
+	if clientIs4 != serverIs4 {
+		return -1, fmt.Errorf("TCPClientUID: IP4/IP6 mismatchh")
+	}
+
 	// Open NETLINK_SOCK_DIAG socket
 	sock, err := sockDiagOpen()
 	if err != nil {
@@ -59,7 +67,20 @@ func TCPClientUID(client, server *net.TCPAddr) (int, error) {
 	rq.hdr.nlmsg_type = C.uint16_t(C.SOCK_DIAG_BY_FAMILY)
 	rq.hdr.nlmsg_flags = C.uint16_t(C.NLM_F_REQUEST)
 
-	rq.data.sdiag_family = C.AF_INET6
+	if clientIs4 {
+		rq.data.sdiag_family = C.AF_INET
+		copy((*[16]byte)(unsafe.Pointer(&rq.data.id.idiag_src))[:],
+			client.IP.To4())
+		copy((*[16]byte)(unsafe.Pointer(&rq.data.id.idiag_dst))[:],
+			server.IP.To4())
+	} else {
+		rq.data.sdiag_family = C.AF_INET6
+		copy((*[16]byte)(unsafe.Pointer(&rq.data.id.idiag_src))[:],
+			client.IP.To16())
+		copy((*[16]byte)(unsafe.Pointer(&rq.data.id.idiag_dst))[:],
+			server.IP.To16())
+	}
+
 	rq.data.sdiag_protocol = C.IPPROTO_TCP
 	rq.data.idiag_states = 1 << C.TCP_ESTABLISHED
 	rq.data.id.idiag_sport = C.uint16_t(toBE16((uint16(client.Port))))
@@ -67,11 +88,6 @@ func TCPClientUID(client, server *net.TCPAddr) (int, error) {
 	rq.data.id.idiag_cookie[0] = C.INET_DIAG_NOCOOKIE
 	rq.data.id.idiag_cookie[1] = C.INET_DIAG_NOCOOKIE
 
-	copy((*[16]byte)(unsafe.Pointer(&rq.data.id.idiag_src))[:],
-		client.IP.To16())
-	copy((*[16]byte)(unsafe.Pointer(&rq.data.id.idiag_dst))[:],
-		server.IP.To16())
-
 	// Send request
 	rqData := (*[unsafe.Sizeof(rq)]byte)(unsafe.Pointer(&rq))
 	rqAddr := &syscall.SockaddrNetlink{Family: syscall.AF_NETLINK}
diff --git a/tcpuid_test.go b/tcpuid_test.go
index e6db60b..d37cb61 100644
--- a/tcpuid_test.go
+++ b/tcpuid_test.go
@@ -16,25 +16,52 @@ import (
 
 // doTestTCPClientUID performs TCPClientUID for the specified
 // network and loopback address
-func doTestTCPClientUID(t *testing.T, network, loopback string) {
+func doTestTCPClientUID(t *testing.T, ip4 bool) {
 	// Do nothing if TCPClientUID is not supported by the platform
 	if !TCPClientUIDSupported() {
 		return
 	}
 
-	// Log local addresses
+	// Log local addresses. Check that we have appropriate
+	// address family support, configured in the system.
+	var haveIP4, haveIP6 bool
+
 	if ift, err := net.Interfaces(); err == nil {
 		for _, ifi := range ift {
 			if addrs, err := ifi.Addrs(); err == nil {
 				t.Logf("%s:", ifi.Name)
 				for _, addr := range addrs {
 					t.Logf("  %s", addr)
+
+					if ipnet, ok := addr.(*net.IPNet); ok {
+						if ipnet.IP.To4() != nil {
+							haveIP4 = true
+						} else {
+							haveIP6 = true
+						}
+					}
 				}
 			}
 		}
 	}
 
+	// Skip incompatible address families
+	if ip4 && !haveIP4 {
+		return
+	}
+
+	if !ip4 && !haveIP6 {
+		return
+	}
+
 	// Create loopback listener -- it gives us a port
+	network := "tcp4"
+	loopback := "127.0.0.1"
+	if !ip4 {
+		loopback = "[::1]"
+		network = "tcp6"
+	}
+
 	l, err := net.Listen(network, loopback+":")
 	if err != nil {
 		t.Fatalf("net.Listen(%q,%q): %s", network, loopback+":", err)
@@ -76,10 +103,10 @@ func doTestTCPClientUID(t *testing.T, network, loopback string) {
 
 // TestTCPClientUIDIp4 performs TCPClientUID test for IPv4
 func TestTCPClientUIDIp4(t *testing.T) {
-	doTestTCPClientUID(t, "tcp", "127.0.0.1")
+	doTestTCPClientUID(t, true)
 }
 
 // TestTCPClientUIDIp6 performs TCPClientUID test for IPv6
 func TestTCPClientUIDIp6(t *testing.T) {
-	doTestTCPClientUID(t, "tcp6", "[::1]")
+	doTestTCPClientUID(t, false)
 }