This is indeed weird. I forked your repo and was able to reproduce it by simple creating a tag, without any other changes to the code. I added -d to make command (inside compile.sh) to get at verbose output. Please see comparison of branch build output (left) and tag build output (right) attached. Note that for the tag build, AppVeyor sets branch name to the tag name and checks out tag instead of commit SHA (see clone steps). Not sure how this could affect your make build. Does detailed log rings some bells?