diff --git a/src/sock.c b/src/sock.c
index 58e128f..b46a0cf 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -721,7 +721,7 @@ int patty_ax25_sock_ack_pending(patty_ax25_sock *sock) {
     for (i=0; i<sock->n_window_tx; i++) {
         struct slot *slot = tx_slot(sock, i);
 
-        if (slot->len > 0 && slot->ack == 0) {
+        if (slot->len > 0 && !slot->ack) {
             ret++;
         }
     }