diff --git a/include/patty/ax25.h b/include/patty/ax25.h
index 37dcd55..251c43c 100644
--- a/include/patty/ax25.h
+++ b/include/patty/ax25.h
@@ -59,6 +59,7 @@ struct _patty_ax25 {
     patty_list * ifaces;
     patty_list * socks;
 
+    int    unix_fdmax;
     fd_set unix_fds;
 
     patty_dict * fd_lookup;
diff --git a/src/if.c b/src/if.c
index 1534e79..b040e37 100644
--- a/src/if.c
+++ b/src/if.c
@@ -240,6 +240,10 @@ int patty_ax25_add_if(patty_ax25 *ax25, patty_ax25_if *iface) {
 
     if ((fd = patty_kiss_tnc_fd_unix(iface->tnc)) >= 0) {
         FD_SET(fd, &ax25->unix_fds);
+
+        if (ax25->unix_fdmax < fd) {
+            ax25->unix_fdmax = fd;
+        }
     }
 
     if (patty_list_append(ax25->ifaces, iface) == NULL) {
@@ -272,6 +276,10 @@ int patty_ax25_delete_if(patty_ax25 *ax25, patty_ax25_if *iface) {
 
     if ((fd = patty_kiss_tnc_fd_unix(iface->tnc)) >= 0) {
         FD_CLR(fd, &ax25->unix_fds);
+
+        if (ax25->unix_fdmax == fd) {
+            ax25->unix_fdmax--;
+        }
     }
 
     return 0;