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

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In my launch file server_gateway.launch, it launch another file whose contents is as follows.

     <launch> 
        <!-- ******************************* Arguments ******************************* --> 
        <arg name="concert_name" default="multinav_concert_scopus"/>
        <arg name="concert_hub_uri" default="http://localhost:6380"/>
        <arg name="gateway_watch_loop_period" default="2"/>
        <arg name="disable_uuids" default="false"/> 

        <!-- ********************************* Hub *********************************** --> 
         <include file="$(find rocon_hub)/launch/hub.launch">  
         <arg name="hub_name" value="$(arg concert_name)" />
         <arg name="hub_port" value="6380" />  </include>  

        <!-- ******************************* Zeroconf ******************************** -->
        <node ns="zeroconf" pkg="zeroconf_avahi" type="zeroconf" name="zeroconf"/> 

        <!-- ******************************** Gateway ******************************** -->  
        <node pkg="rocon_gateway" type="gateway.py" name="gateway">    
          <rosparam command="load" file="$(find rocon_gateway)/param/default.yaml"/>  
          <rosparam command="load" file="$(find rocon_gateway)/param/default_blacklist.yaml"/>   
          <rosparam command="load" file="$(find scopus_gateway)/param/server_advertisements.yaml" />   
          <rosparam command="load" file="$(find scopus_gateway)/param/server_flips.yaml" /> 
          <param name="hub_uri" value="$(arg concert_hub_uri)"/>    
          <param name="name" value="$(arg concert_name)"/>  
          <param name="firewall" value="false"/>
          <param name="watch_loop_period" value="$(arg gateway_watch_loop_period)"/> 
          <param name="hub_whitelist" value=""/>  
          <param name="disable_uuids" value="$(arg disable_uuids)"/>  
        </node> 
</launch>

But when running roslaunch server_gateway.launch, it comes out error:

load_parameters: unable to set parameters (last param was [/concert/gateway/hub_uri=http://localhost:6380]): cannot marshal None unless allow_none is enabled

Traceback (most recent call last):  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/__init__.py", line 279, in main    p.start()  

File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/parent.py", line 268, in start    self.runner.launch()

File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/launch.py", line 644, in launch    self._setup()  

File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/launch.py", line 631, in _setup    self._load_parameters() 

File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/launch.py", line 328, in _load_parameters    r  = param_server_multi() 

File "/usr/lib/python2.7/xmlrpclib.py", line 997, in __call__    return MultiCallIterator(self.__server.system.multicall(marshalled_list))  

File "/usr/lib/python2.7/xmlrpclib.py", line 1224, in __call__    return self.__send(self.__name, args)  

File "/usr/lib/python2.7/xmlrpclib.py", line 1572, in __request    allow_none=self.__allow_none)  

File "/usr/lib/python2.7/xmlrpclib.py", line 1085, in dumps    data = m.dumps(params)  

File "/usr/lib/python2.7/xmlrpclib.py", line 632, in dumps    dump(v, write)  

File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump    f(self, value, write)  

File "/usr/lib/python2.7/xmlrpclib.py", line 714, in dump_array    dump(v, write)  

File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump    f(self, value, write)  

File "/usr/lib/python2.7/xmlrpclib.py", line 735, in dump_struct    dump(v, write)  

File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump    f(self, value, write)  

File "/usr/lib/python2.7/xmlrpclib.py", line 714, in dump_array    dump(v, write)  

File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump    f(self, value, write)  

File "/usr/lib/python2.7/xmlrpclib.py", line 658, in dump_nil    raise TypeError, "cannot marshal None unless 
allow_none is ...