diff --git a/adafruit_husb238.py b/adafruit_husb238.py
index 4743222..27ccb9d 100644
--- a/adafruit_husb238.py
+++ b/adafruit_husb238.py
@@ -143,6 +143,7 @@ class Adafruit_HUSB238:
     _PDO_RESPONSE_CODES = [
         "NO RESPONSE",
         None,  # Success
+        None,  # Reserved
         "INVALID COMMAND OR ARGUMENT",
         "COMMAND NOT SUPPORTED",
         "TRANSACTION FAILED, NO GOOD CRC",