How to solve the error "cannot marshal None unless allow_none is enabled ”

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Hi, I am new to ros. we are trying to interface the dynamixel Pro-42 series actuator to our hardware. we are following steps mentioned in tutorial "http://wiki.ros.org/dynamixel_controllers/Tutorials/ConnectingToDynamixelBus"

the problem is when we give command

roslaunch my_dynamixel_tutorial controller_manager.launch

on console it prints two messages that are:

load_parameter : Last successfully loaded parameter (/dev/ttyUSB0)
  "cannot marshal None unless allow_none is enabled"
File "/usr/lib/python2.7/xmlrpclib.py", line 664, in dump_nil
  raise TypeError, "cannot marshal None unless allow_none is enabled
"TypeError: cannot marshal None unless allow_none is enabled

How do i solve this issue..

<!-- -*- mode: XML -*- -->

<launch>
<node name="dynamixel_manager" pkg="dynamixel_controllers" type="controller_manager.py" required="true" output="screen">
<rosparam>
namespace: dxl_manager
serial_ports:
pan_tilt_port:
port_name: "/dev/ttyUSB0"
baud_rate: 57600
min_motor_id: 1
max_motor_id: 25
update_rate: 20
</rosparam>
</node>
</launch>

thanks in advance..