TypeError: 'NoneType' object is not subscriptable

ERROR: Could not build app org.xcsoar due to unknown error: Traceback (most recent call last):
  File "/home/vagrant/fdroidserver/fdroidserver/build.py", line 1101, in main
    options.onserver, options.refresh):
  File "/home/vagrant/fdroidserver/fdroidserver/build.py", line 867, in trybuild
    build_local(app, build, vcs, build_dir, output_dir, log_dir, srclib_dir, extlib_dir, tmp_dir, force, onserver, refresh)
  File "/home/vagrant/fdroidserver/fdroidserver/build.py", line 515, in build_local
    count = scanner.scan_source(build_dir, build)
  File "/home/vagrant/fdroidserver/fdroidserver/scanner.py", line 277, in scan_source
    warnproblem('possible binary', path_in_build_dir)
  File "/home/vagrant/fdroidserver/fdroidserver/scanner.py", line 161, in warnproblem
    json_per_build['warnings'].append([what, path_in_build_dir])
TypeError: 'NoneType' object is not subscriptable

Seen on main build server at 1be8788f in com.tutpro.baresip/lastbuild 125 or https://f-droid.org/wiki/page/org.xcsoar/lastbuild_116