enforce list data types in YAML metadata files

I can't get past the following (have tried deleting the apkcache). Although I can't see what change would have caused this...

CRITICAL: Unknown exception found!
Traceback (most recent call last):
  File "/home/fbuild/fdroidserver/fdroid", line 164, in <module>
    main()
  File "/home/fbuild/fdroidserver/fdroid", line 159, in main
    raise e
  File "/home/fbuild/fdroidserver/fdroid", line 138, in main
    mod.main()
  File "/home/fbuild/fdroidserver/fdroidserver/update.py", line 2052, in main
    index.make(apps, sortedids, apks, repodirs[0], False)
  File "/home/fbuild/fdroidserver/fdroidserver/index.py", line 140, in make
    fdroid_signing_key_fingerprints)
  File "/home/fbuild/fdroidserver/fdroidserver/index.py", line 463, in make_v0
    app.AntiFeatures.extend(apklist[0]['antiFeatures'])
AttributeError: 'str' object has no attribute 'extend'