Seems to have been fixed in Arduino Builder 1.3.21. I don't have access to Sierra right now. Can you please give the attached arduino-builder a try? Arduino builder is located in Energia.app/Contents/Java/   Note that this arduino-builder is a slightly modified version of the original arduino-builder. Reason is in the pull-request here: https://github.com/arduino/arduino-builder/pull/119     arduino-builder.zip