data['vector_layers'] is undefined

[pitommasi]

I am trying to serve tiles from a mbtiles created with tippecanoe, projection EPSG:3857.
The mbtiles are shown correctly in MapTiler.

I've set up a tileserver.php app but the tiles are not loaded (i.e.: no network requests in the dev panel).
In order to debug, I've copy pasted the code that the tileserver.php provides for Openlayers (I need the tiles to work with OL3) and I managed to see that it fails here:

data['vector_layers'].forEach(function(el) {

because data['vector_layers'] is undefined...

any thought?