Niceness value in global config seems to be ignored

In the global section of my kde-builder.yaml I have:

  niceness: "19"

but when I build anything the build processes have the default niceness of 10.

Removing the double-quotes produces a python error “expected string or bytes-like object, got ‘int’” (shouldn’t it be expecting an int?)

Passing --nice=19 on the command line works as expected.