Index: head/lib/libc/net/if_indextoname.c
===================================================================
--- head/lib/libc/net/if_indextoname.c
+++ head/lib/libc/net/if_indextoname.c
@@ -66,6 +66,11 @@
 	struct ifaddrs *ifaddrs, *ifa;
 	int error = 0;
 
+	if (ifindex == 0) {
+		errno = ENXIO;
+		return(NULL);
+	}
+
 	if (getifaddrs(&ifaddrs) < 0)
 		return(NULL);	/* getifaddrs properly set errno */