Problem to add a new format

Hello,

I tried to add a new format to my story : https://github.com/cauli/TwineJson, but Twinery refused to load it.
I also tried with and without HTTPS and with an another format : https://github.com/lazerwalker/twison.

I noticed an error log on the JS console when I click 'Add' :

ReferenceError: existing is not defined      twine.js:1:3784

I made test with Firefox 51 and Chrome 56.