Drilldown Choropleth - Drilldown Issue

Hi,

I'm facing an issue with Drilldown Choropleth and can't seem to find an answer :

I'm trying to display 4 countys and all of their cities, like that :

As you can see, this is the same file with the same parameters.

It works just fine those arrows :

But when i click on a county to show cities from this specific county, i got a blank page

The only exemples i could find all were with outdated links (404), is there a link i need to create between my two TOPO JSON files ?

Here are the links to my TOPO JSON, if you want to check them out :
https://github.com/BOYERAlexis/TOPOJSON
(counties are Départements & cities are Communes)