Building mDot library -ar command includes no object files

The ar command ends up being arm-none-eabi-ar rcs .pioenvs/mDot/lib/libmDot_ID308.a with no object files specified. Needless to say, downstream, the linker fails with undefined reference to `mDot::getInstance()'.

Any idea why an archive command would not include any object files? And what to do about it?

Thanks!

Did you try to pass extra flags? build_flags = -lmDot