Power Bi Embedded customData, can we pass null role?

we are using below method, and pass null as role and custom data as user value; but it fails with bad request and does it work only using azure analysis server role?

public EffectiveIdentity(string username, IList<string> datasets, IList<string> roles = null, string customData = null);

sample app have this note

 Comma separeted roles, optional for SSAS, mandatory for roles defined in pbix