ROS can't read json file

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Hello,

I am trying to communicate a non ros-program to ros kinetic via MQTT Broker. mqtt_bridge node make it easier and I already configurated it properly. When I launch roslaunch mqtt_bridge demo.launch it runs very well without any problem. As soon I start the communication from a non ros-programm to ros, the launch file throw this error message multiple times "Message type std_msgs/String does not have a field position".

The message program looks like this:

import json
import paho.mqtt.client as mqtt
import time

client = mqtt.Client()
client.connect(192.168.1.111, 1883, 60)
msg = {
    'position': [
        {
            'x_value' : '5',
            'y_value' : '2.6'
        }
    ]
 }
 json_msg=json.dumps(msg)
 channel = "ZIRP/FTS/N1/Pos"
 client = mqttClient
 client.publish(channel, json_msg)

The code is written in python and run in a non ros PC. The code doesn't throw any problem and the mqtt broker receive the message successfully. The sent data structure is string and probably in array but I'm not sure.

The configuration in mqtt_bridge node:

mqtt:
  client:
    protocol: 4      # MQTTv311
  connection:
    host: 192.168.1.101
    port: 1883
    keepalive: 60
  private_path: device/001

bridge:
   # echo back
   - factory: mqtt_bridge.bridge:RosToMqttBridge
      msg_type: std_msgs.msg:String
      topic_from: /echo
      topic_to: ZIRP/FTS/N1/Pos
   - factory: mqtt_bridge.bridge:MqttToRosBridge
     msg_type: std_msgs.msg:String
     topic_from: ZIRP/FTS/N1/Pos
     topic_to: /echo

Any idea why ros can't read the message?