diff --git a/miniupnpd/natpmp.c b/miniupnpd/natpmp.c
index 2bacb8f..be37e29 100644
--- a/miniupnpd/natpmp.c
+++ b/miniupnpd/natpmp.c
@@ -293,8 +293,9 @@ void ProcessIncomingNATPMPPacket(int s, unsigned char *msg_buff, int len,
 					}
 					any_eport_allowed = 1;	/* at lease one eport is allowed */
 #ifdef CHECK_PORTINUSE
-					if (port_in_use(ext_if_name, eport, proto, senderaddrstr, iport)) {
-						syslog(LOG_INFO, "port %hu protocol %s already in use", eport, (proto==IPPROTO_TCP)?"tcp":"udp");
+					if (port_in_use(ext_if_name, eport, proto, senderaddrstr, iport) > 0) {
+						syslog(LOG_INFO, "port %hu protocol %s already in use",
+						       eport, (proto==IPPROTO_TCP)?"tcp":"udp");
 						eport++;
 						if(eport == 0) eport++; /* skip port zero */
 						continue;
diff --git a/miniupnpd/portinuse.c b/miniupnpd/portinuse.c
index c9b4849..278faa7 100644
--- a/miniupnpd/portinuse.c
+++ b/miniupnpd/portinuse.c
@@ -1,4 +1,4 @@
-/* $Id $ */
+/* $Id: $ */
 /* MiniUPnP project
  * (c) 2007-2014 Thomas Bernard
  * http://miniupnp.free.fr/ or http://miniupnp.tuxfamily.org/
@@ -83,13 +83,14 @@ port_in_use(const char *if_name,
 		ip_addr.s_addr = 0;
 
 	syslog(LOG_DEBUG, "Check protocol %s for port %d on ext_if %s %s, %08X",
-		(proto==IPPROTO_TCP)?"tcp":"udp", eport, if_name, ip_addr_str, (unsigned)ip_addr.s_addr);
+	    (proto==IPPROTO_TCP)?"tcp":"udp", eport, if_name,
+	    ip_addr_str, (unsigned)ip_addr.s_addr);
 
 #ifdef __linux__
 	f = fopen((proto==IPPROTO_TCP)?tcpfile:udpfile, "r");
 	if (!f) {
 		syslog(LOG_ERR, "cannot open %s", (proto==IPPROTO_TCP)?tcpfile:udpfile);
-		return 0;
+		return -1;
 	}
 
 	while (fgets(line, 255, f)) {
diff --git a/miniupnpd/portinuse.h b/miniupnpd/portinuse.h
index fddc7c8..01842f6 100644
--- a/miniupnpd/portinuse.h
+++ b/miniupnpd/portinuse.h
@@ -1,4 +1,4 @@
-/* $Id $ */
+/* $Id: $ */
 /* MiniUPnP project
  * http://miniupnp.free.fr/ or http://miniupnp.tuxfamily.org/
  * (c) 2006-2014 Thomas Bernard
@@ -12,7 +12,8 @@
 /* portinuse()
  * determine wether a port is already in use
  * on a given interface.
- * returns: 0 not in use, 1 in use */
+ * returns: 0 not in use, > 0 in use
+ *         -1 in case of error */
 int
 port_in_use(const char *if_name,
             unsigned port, int proto,
diff --git a/miniupnpd/upnpredirect.c b/miniupnpd/upnpredirect.c
index 0139a7d..5a50c6d 100644
--- a/miniupnpd/upnpredirect.c
+++ b/miniupnpd/upnpredirect.c
@@ -296,8 +296,9 @@ upnp_redirect(const char * rhost, unsigned short eport,
 			return -2;
 		}
 #ifdef CHECK_PORTINUSE
-	} else if (port_in_use(ext_if_name, eport, proto, iaddr, iport)) {
-		syslog(LOG_INFO, "port %hu protocol %s already in use", eport, protocol);
+	} else if (port_in_use(ext_if_name, eport, proto, iaddr, iport) > 0) {
+		syslog(LOG_INFO, "port %hu protocol %s already in use",
+		       eport, protocol);
 		return -2;
 #endif /* CHECK_PORTINUSE */
 	} else {