Your problem is that whoever designed that Serial class is incompetent and should be shot and their body defiled in unspeakable ways.

The C-language varargs concept and the C++ object model are not compatible. C++ provides varargs only for backwards compatibility with legacy C libraries. New APIs written in C++ should not be using it because passing a C++ object through it is undefined behaviour. No amount of rationalizing (as all the comments downthread are doing) is of any value: undefined behavior is undefined.