ROS master crashing due to XMLRPC error

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I restarted my robot, running ROS Kinetic on a Raspberry Pi, and I'm finding that the master node is immediately crashing on startup with the error:

auto-starting new master
process[master]: started with pid [7708]
ROS_MASTER_URI=http://localhost:11311

setting /run_id to 5dab8662-0754-11e8-a585-b827eb09ee17
process[rosout-1]: started with pid [7721]
started core service [/rosout]
load_parameters: unable to set parameters (last param was [/head_arduino/serial_node/baud=115200]): cannot marshal None unless allow_none is enabled
Traceback (most recent call last):
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/__init__.py", line 307, in main
    p.start()
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/parent.py", line 279, in start
    self.runner.launch()
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/launch.py", line 654, in launch
    self._setup()
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/launch.py", line 641, in _setup
    self._load_parameters()
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/launch.py", line 338, in _load_parameters
    r  = param_server_multi()
  File "/usr/lib/python2.7/xmlrpclib.py", line 1006, in __call__ 
    return MultiCallIterator(self.__server.system.multicall(marshalled_list))
  File "/usr/lib/python2.7/xmlrpclib.py", line 1243, in __call__ 
    return self.__send(self.__name, args)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1596, in __request
    allow_none=self.__allow_none)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1094, in dumps
    data = m.dumps(params)
  File "/usr/lib/python2.7/xmlrpclib.py", line 638, in dumps
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 660, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 719, in dump_array
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 660, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 741, in dump_struct
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 660, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 719, in dump_array
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 660, in __dump
    f(self, value, write)
  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
[rosout-1] killing on exit
[master] killing on exit

Looking in my ~/.ros/latest/latest/roslaunch-rae-8050.log I also see the lines:

[roslaunch][INFO] '2018-02-01 08:55:26': load_parameters starting ...
[roslaunch][ERROR] '2018-02-01 08:55:26': load_parameters: unable to set parameters (last param was [/head_arduino/serial_node/baud=115200]): cannot marshal None unless allow_none is enabled

What's causing this?

My node /head_arduino/serial_node/baud=115200 is just a stock rosserial serial_node.py instance launched with:

<launch>
    <group ns="head_arduino">
        <node pkg="rosserial_python" type="serial_node.py" name="serial_node" output="screen">
            <param name="~port" value="/dev/ttyACM0" />
            <param name="~baud" value="115200" />
        </node>
    </group>
</launch>

Clearly, none of its parameters are None.

Googling the error suggests it's a generic exception thrown by xmlrpclib when an XMLRPC endpoint receives a bad ... (more)