diff --git a/include/patty/ax25/sock.h b/include/patty/ax25/sock.h
index 6a44f50..965d1bb 100644
--- a/include/patty/ax25/sock.h
+++ b/include/patty/ax25/sock.h
@@ -17,7 +17,7 @@
    | PATTY_AX25_PARAM_HDLC_XADDR | PATTY_AX25_PARAM_HDLC_MODULO_8 \
    | PATTY_AX25_PARAM_HDLC_TEST  | PATTY_AX25_PARAM_HDLC_SYNC_TX)
 
-#define PATTY_AX25_SOCK_DEFAULT_I_LEN   128
+#define PATTY_AX25_SOCK_DEFAULT_I_LEN   127 /* I field total, minus proto */
 #define PATTY_AX25_SOCK_DEFAULT_WINDOW    4
 #define PATTY_AX25_SOCK_DEFAULT_RETRY    10
 #define PATTY_AX25_SOCK_DEFAULT_ACK    3000