When builder is uploading empty file (I've tried run it on empty tag), fastUpload will skip even first uploading cycle. It will traceback later when running checkUpload on file which was never created on hub and returning None in such situation. So, few problems here:

# builder probably should be able to upload empty file (random placeholders, etc.)
# fastUpload should fail better when file failed to upload

I'll submit a fix PR tomorrow.