Atlas - blocks.json

Hi people, I used to have a array of type:single sources in my blocks.json atlas file, but I'm now trying to just include the whole texture directory under my namespace.
Yet, my json seems not to work.
    "sources": [
        {
            "type": "directory",
            "source": "ulg:custom",
            "prefix": "ulg:custom/"
        }
    ]
}``` nor ```{
    "sources": [
        {
            "type": "directory",
            "source": "ulg",
            "prefix": "ulg:"
        }
    ]
}``` (wild attempt)
work.

From my console I get the
Invalid path textures/ulg:custom Invalid segment 'ulg:custom' in path 'textures/ulg:custom'
So what's the way to indicate my custom namespace as path?

pls help 🙂