How to call a service with a char argument

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How do you pass a char to rosservice call?

I have a srv file like:

uint8 id
string data
---

exposed as /myservice which I'd like to call like rosservice call /myservice m '!!str mydata' but this gives me a Python traceback to the autogenerated code for my srv class:

UnboundLocalError: local variable '_x' referenced before assignment

presumably because it doesn't convert "m" to the uint8 value 109. The docs declare char is deprecated because it's essentially the same thing as uint8.

Is there a way to explicitly declare this type, or do I have to pass in a literal int value?