diff --git a/modbus/client-serial-modbus.py b/modbus/client-serial-modbus.py
index c9cf3c0..6768a5f 100644
--- a/modbus/client-serial-modbus.py
+++ b/modbus/client-serial-modbus.py
@@ -74,6 +74,9 @@ def run_sync_client(udp_addr, udp_port_rx, udp_port_tx, serial_port, serial_baud
         if 'data' not in message:
             message['data'] = []
 
+        if 'unit' not in message:
+            message['unit'] = unit_num
+
         rc = False
         if message['code'] == 'write':
             data = message['data']