diff --git a/src/ax25.c b/src/ax25.c
index 5eeaa93..29a5de2 100644
--- a/src/ax25.c
+++ b/src/ax25.c
@@ -65,6 +65,10 @@ int patty_ax25_pton(const char *callsign,
         }
     }
 
+    if (ssid > 15) {
+        goto error_invalid_callsign;
+    }
+
     if (c == '-') {
         goto error_invalid_callsign;
     }