ros2 launch arg bug with string?

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In the example launch file below:

<launch>
  <arg name = "foo" default = "a:"/>
  <node pkg = "demo_nodes_py" exec = "listener" output = "screen">
    <param name = "foo"  type = "str" value = "$(var foo)" />
  </node>
</launch>

I get the following error which I don't get if there is not the trailing ":". The error message should also be more informative, maybe provide the arg name and value.

aravind@carrot:ros2_examples$ ros2 launch test.launch.xml
[INFO] [launch]: All log files can be found below /home/aravind/.ros/log/2020-08-07-15-11-27-621475-carrot-25030
[INFO] [launch]: Default logging verbosity is set to INFO
[ERROR] [launch]: Caught exception in launch (see debug for traceback): Allowed value types are bytes, bool, int, float, str, Sequence[bool], Sequence[int], Sequence[float], Sequence[str]. Got <class 'dict'>.

When I replace it with an integer, I get the following error when I have a C++ node that tries to access the "foo" parameter as a string.

[talker-1] terminate called after throwing an instance of 'rclcpp::ParameterTypeException'
[talker-1]   what():  expected [string] got [integer]

There seems to be implicit conversion here. How can I pass arbitrary strings without conversion?

Thanks!