dub is building docs of dependencies

[Jedzia via Digitalmars-d-learn]

dmd2-2.083.0, win
minimal dub init
with dub.json:
{
        "description": "A minimal D application.",
        "dependencies": {
                "iz": "~>0.6.23"
        },
        "authors": [
                "Jedzia"
        ],
        "copyright": "Copyright © 2018, Jedzia",
        "license": "MIT",
        "name": "testdoc"
}
and `dub build -b docs`

Is it normal that dub builds the docs of dependencies (iz library 
in that case).
And if, how to build only the docs of my own project?