redux-persist - how do you blacklist/whitelist nested state

So I have a credentials object which contains a password and a username

payload: Object
  credentials: Object
    password: ""
    username: ""

and I want to blacklist password in the reducer configuration, something like

const authPersistConfig = {
    key: 'AuthReducer',
    storage: storage,
    blacklist: ['credentials.password']
};

If I use this code, both the credential states end up being blacklisted. I want to persist the username but not the password.

It might be that redux-persist only persists top-level state or it might be a syntax error, or something else entirely – any ideas?

Many thanks