Serial Port read failure: device reports readiness to read but returned no data (device disconnected?)

edit

I have been using rosserial connect to Mbed and the node was working well, and recently there was a few times of software update. After the update I cannot run the node and it give the following information. My Mbed was operating well and the I am currently using Ubuntu14.04 ros indigo

[INFO] [WallTime: 1445252210.206610] ROS Serial Python Node

[INFO] [WallTime: 1445252210.216847] Connecting to /dev/ttyACM0 at 57600 baud

[WARN] [WallTime: 1445252212.329202] Serial Port read failure: device reports readiness to read but returned no data (device disconnected?)

[INFO] [WallTime: 1445252212.329920] Packet Failed :  Failed to read msg data
Traceback (most recent call last):
  File "/home/ju/catkin_ws/src/rosserial/rosserial_python/nodes/serial_node.py", line 82, in <module>
    client.run()
  File "/home/ju/catkin_ws/src/rosserial/rosserial_python/src/rosserial_python/SerialClient.py", line 472, in run
    rospy.loginfo("msg len is %d",len(msg))
UnboundLocalError: local variable 'msg' referenced before assignment