diff --git a/v5_field_control.ino b/v5_field_control.ino
index 4b564bb..32d2662 100644
--- a/v5_field_control.ino
+++ b/v5_field_control.ino
@@ -245,9 +245,13 @@ void parse_poll(uint8_t* pkt, size_t len) {
 }
 
 void on_rx(void) {
-  uint8_t pkt[4];
+  uint8_t pkt[12];
   int n = Serial.available();
-  if (n < 4) {
+  if (n != 12) {
+    Serial.printf("Wrong number of bytes on serial port %d/12\n", n);
+    for (int i = 0; i < n; i++) {
+      Serial.read();
+    }
     return;
   }
   int read = Serial.readBytes(pkt, n);