Aurelia's computedFrom with an object

I have a state object which has a property session, this property can either be an object or null.

I don't want to have to dirty-check for the isSessionActive() getter, so I'd like to use computedFrom(). However, computerFrom() doesn't seem to fire when this object changes, unless it was undefined previously.

Can I do this without a dedicated isSessionActive boolean property on my state Store?

@autoinject
export class Home {
    firstName: string = "user";
    private state: State;

    constructor(private store: Store) {
       store.state.subscribe(
            response => this.state = response
       )
    }

    @computedFrom('state.activeSession')
    get isSessionActive() {
        return this.state.activeSession !== null;
    }
}