diff --git a/modbus/client-serial-modbus.py b/modbus/client-serial-modbus.py
index 73d6375b..c9cf3c0f 100644
--- a/modbus/client-serial-modbus.py
+++ b/modbus/client-serial-modbus.py
@@ -87,14 +87,14 @@ def run_sync_client(udp_addr, udp_port_rx, udp_port_tx, serial_port, serial_baud
             rq = client.read_holding_registers(data[0], data[1], unit=unit_num)
             rc = rq.isError()
             if not rc:
-                tx_data = json_encoder.encode({'code': 'response', 'data': rq.registers})
+                tx_data = json_encoder.encode({'code': 'response', 'data': [data[0]] + rq.registers})
                 sock.sendto(bytes(tx_data, 'utf-8'), (udp_addr, udp_port_tx))
         elif message['code'] == 'read_input':
             data = message['data']
             rq = client.read_input_registers(data[0], data[1], unit=unit_num)
             rc = rq.isError()
             if not rc:
-                tx_data = json_encoder.encode({'code': 'response', 'data': rq.registers})
+                tx_data = json_encoder.encode({'code': 'response', 'data': [data[0]] + rq.registers})
                 sock.sendto(bytes(tx_data, 'utf-8'), (udp_addr, udp_port_tx))
         elif message['code'] == '<unknown>':
             rc = False