ConstantParameterDefault being internal

[Falco20019]

I tried to define a parameter configurable by configuration with a fallback value:
var baseTagParam = builder.AddParameter("base-tag", new ConstantParameterDefault(() => "latest"));

But ConstantParameterDefault is declared whereas GenerateParameterDefault is public. AddParameterFromConfiguration also doesn't offer a fallback value to be used. Am I missing something simple here?