diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index d062c1d8ce..8fa9aa797f 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -1463,7 +1463,7 @@ ident_inet(hbaPort *port)
 
 	sock_fd = socket(ident_serv->ai_family, ident_serv->ai_socktype,
 					 ident_serv->ai_protocol);
-	if (sock_fd < 0)
+	if (sock_fd == PGINVALID_SOCKET)
 	{
 		ereport(LOG,
 				(errcode_for_socket_access(),
@@ -1543,7 +1543,7 @@ ident_inet(hbaPort *port)
 					ident_response)));
 
 ident_inet_done:
-	if (sock_fd >= 0)
+	if (sock_fd != PGINVALID_SOCKET)
 		closesocket(sock_fd);
 	pg_freeaddrinfo_all(remote_addr.addr.ss_family, ident_serv);
 	pg_freeaddrinfo_all(local_addr.addr.ss_family, la);
@@ -2361,7 +2361,7 @@ CheckRADIUSAuth(Port *port)
 	packet->length = htons(packet->length);
 
 	sock = socket(serveraddrs[0].ai_family, SOCK_DGRAM, 0);
-	if (sock < 0)
+	if (sock == PGINVALID_SOCKET)
 	{
 		ereport(LOG,
 				(errmsg("could not create RADIUS socket: %m")));
diff --git a/src/backend/libpq/ip.c b/src/backend/libpq/ip.c
index 7e8fc785bb..acdbab0049 100644
--- a/src/backend/libpq/ip.c
+++ b/src/backend/libpq/ip.c
@@ -547,7 +547,7 @@ pg_foreach_ifaddr(PgIfAddrCallback callback, void *cb_data)
 	int			error;
 
 	sock = WSASocket(AF_INET, SOCK_DGRAM, 0, 0, 0, 0);
-	if (sock == SOCKET_ERROR)
+	if (sock == INVALID_SOCKET)
 		return -1;
 
 	while (n_ii < 1024)
@@ -670,7 +670,7 @@ pg_foreach_ifaddr(PgIfAddrCallback callback, void *cb_data)
 				total;
 
 	sock = socket(AF_INET, SOCK_DGRAM, 0);
-	if (sock == -1)
+	if (sock == PGINVALID_SOCKET)
 		return -1;
 
 	while (n_buffer < 1024 * 100)
@@ -711,7 +711,7 @@ pg_foreach_ifaddr(PgIfAddrCallback callback, void *cb_data)
 #ifdef HAVE_IPV6
 	/* We'll need an IPv6 socket too for the SIOCGLIFNETMASK ioctls */
 	sock6 = socket(AF_INET6, SOCK_DGRAM, 0);
-	if (sock6 == -1)
+	if (sock6 == PGINVALID_SOCKET)
 	{
 		free(buffer);
 		close(sock);
@@ -788,10 +788,10 @@ pg_foreach_ifaddr(PgIfAddrCallback callback, void *cb_data)
 	char	   *ptr,
 			   *buffer = NULL;
 	size_t		n_buffer = 1024;
-	int			sock;
+	pgsocket	sock;
 
 	sock = socket(AF_INET, SOCK_DGRAM, 0);
-	if (sock == -1)
+	if (sock == PGINVALID_SOCKET)
 		return -1;
 
 	while (n_buffer < 1024 * 100)
diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index 1eae183e33..0179451f08 100644
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -392,7 +392,7 @@ StreamServerPort(int family, char *hostName, unsigned short portNumber,
 				break;
 		}
 
-		if ((fd = socket(addr->ai_family, SOCK_STREAM, 0)) < 0)
+		if ((fd = socket(addr->ai_family, SOCK_STREAM, 0)) == PGINVALID_SOCKET)
 		{
 			ereport(LOG,
 					(errcode_for_socket_access(),
@@ -632,7 +632,7 @@ StreamConnection(pgsocket server_fd, Port *port)
 	port->raddr.salen = sizeof(port->raddr.addr);
 	if ((port->sock = accept(server_fd,
 							 (struct sockaddr *) & port->raddr.addr,
-							 &port->raddr.salen)) < 0)
+							 &port->raddr.salen)) == PGINVALID_SOCKET)
 	{
 		ereport(LOG,
 				(errcode_for_socket_access(),
diff --git a/src/backend/port/win32/socket.c b/src/backend/port/win32/socket.c
index 4f1099f6cf..adc0e02335 100644
--- a/src/backend/port/win32/socket.c
+++ b/src/backend/port/win32/socket.c
@@ -132,7 +132,7 @@ int
 pgwin32_waitforsinglesocket(SOCKET s, int what, int timeout)
 {
 	static HANDLE waitevent = INVALID_HANDLE_VALUE;
-	static SOCKET current_socket = -1;
+	static SOCKET current_socket = INVALID_SOCKET;
 	static int	isUDP = 0;
 	HANDLE		events[2];
 	int			r;
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index bb771a63f2..b573fd82b6 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -2169,7 +2169,7 @@ ConnCreate(int serverFd)
 
 	if (StreamConnection(serverFd, port) != STATUS_OK)
 	{
-		if (port->sock >= 0)
+		if (port->sock != PGINVALID_SOCKET)
 			StreamClose(port->sock);
 		ConnFree(port);
 		return NULL;
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index d53c41f6a3..51d4de419b 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -1632,8 +1632,23 @@ keep_going:						/* We will come back to here until there is
 					conn->raddr.salen = addr_cur->ai_addrlen;
 
 					/* Open a socket */
-					conn->sock = socket(addr_cur->ai_family, SOCK_STREAM, 0);
-					if (conn->sock < 0)
+					{
+						/*
+						 * While we use 'pgsocket' as the socket type in the
+						 * backend, we use 'int' for libpq socket values.
+						 * This requires us to map PGINVALID_SOCKET to -1
+						 * on Windows.
+						 * See http://msdn.microsoft.com/en-us/library/windows/desktop/ms740516%28v=vs.85%29.aspx
+						 */
+						pgsocket sock = socket(addr_cur->ai_family, SOCK_STREAM, 0);
+#ifdef WIN32
+						if (sock == PGINVALID_SOCKET)
+							conn->sock = -1;
+						else
+#endif
+							conn->sock = sock;
+					}
+					if (conn->sock == -1)
 					{
 						/*
 						 * ignore socket() failure if we have more addresses
@@ -3136,7 +3151,7 @@ internal_cancel(SockAddr *raddr, int be_pid, int be_key,
 				char *errbuf, int errbufsize)
 {
 	int			save_errno = SOCK_ERRNO;
-	int			tmpsock = -1;
+	pgsocket	tmpsock = PGINVALID_SOCKET;
 	char		sebuf[256];
 	int			maxlen;
 	struct
@@ -3149,7 +3164,7 @@ internal_cancel(SockAddr *raddr, int be_pid, int be_key,
 	 * We need to open a temporary connection to the postmaster. Do this with
 	 * only kernel calls.
 	 */
-	if ((tmpsock = socket(raddr->addr.ss_family, SOCK_STREAM, 0)) < 0)
+	if ((tmpsock = socket(raddr->addr.ss_family, SOCK_STREAM, 0)) == PGINVALID_SOCKET)
 	{
 		strlcpy(errbuf, "PQcancel() -- socket() failed: ", errbufsize);
 		goto cancel_errReturn;
@@ -3220,7 +3235,7 @@ cancel_errReturn:
 				maxlen);
 		strcat(errbuf, "\n");
 	}
-	if (tmpsock >= 0)
+	if (tmpsock != PGINVALID_SOCKET)
 		closesocket(tmpsock);
 	SOCK_ERRNO_SET(save_errno);
 	return FALSE;
@@ -5300,6 +5315,15 @@ PQerrorMessage(const PGconn *conn)
 	return conn->errorMessage.data;
 }
 
+/*
+ * In Windows, socket values are unsigned, and an invalid socket value
+ * (INVALID_SOCKET) is ~0, which equals -1 in comparisons (with no compiler
+ * warning). Ideally we would return an unsigned value for PQsocket() on
+ * Windows, but that would cause the function's return value to differ from
+ * Unix, so we just return -1 for invalid sockets.
+ * http://msdn.microsoft.com/en-us/library/windows/desktop/cc507522%28v=vs.85%29.aspx
+ * http://stackoverflow.com/questions/10817252/why-is-invalid-socket-defined-as-0-in-winsock2-h-c
+ */
 int
 PQsocket(const PGconn *conn)
 {
diff --git a/src/interfaces/libpq/libpq-int.h b/src/interfaces/libpq/libpq-int.h
index 22bbe4a48e..ee975d41fa 100644
--- a/src/interfaces/libpq/libpq-int.h
+++ b/src/interfaces/libpq/libpq-int.h
@@ -364,6 +364,7 @@ struct pg_conn
 	PGnotify   *notifyTail;		/* newest unreported Notify msg */
 
 	/* Connection data */
+	/* See PQconnectPoll() for how we use 'int' and not 'pgsocket'. */
 	int			sock;			/* Unix FD for socket, -1 if not connected */
 	SockAddr	laddr;			/* Local address */
 	SockAddr	raddr;			/* Remote address */