Impossible to plot Choropleth map

[mathgim]

I try to plot a Choropleth map to display "Departements" in France (kind of regions) using this geojson : https://github.com/gregoiredavid/france-geojson/blob/master/departements.geojson. But it seems not to work. I have a "Departements" table with a dept column containing Departement codes and another text column for my z parameter. I tried many different things in Custom but none of this seems to be working.
Maybe a bug in the Custom feature?