Typescript Strict Mode: having error on BehaviouralSubject (null)

Question

I'm trying to follow the documentation on dynamic query from the AngularFireStore documentation, here I'm just starting and upon typing the behaviourSubject on my constructor.

I know this is just a typescript rule matter, but I am new and also confused what's wrong here. Disabling strict mode does it but I don't want to do that.

export class MembersSequencingComponent implements OnInit {
    customers$ : Observable<Customer[]>; // instead of using an interface I've used a model
    zoneFilters$: BehaviorSubject<Customer|null>;

constructor(afs: AngularFirestore) {
    
    this.zoneFilters$ = new BehaviorSubject(null); >> //typescript error happens here


   }

 
}

However, my typescript is giving me an error.

Type 'BehaviorSubject<null>' is not assignable to type 'BehaviorSubject<Customer | null>'.
  Types of property 'observers' are incompatible.
    Type 'Observer<null>[]' is not assignable to type 'Observer<Customer | null>[]'.
      Type 'Observer<null>' is not assignable to type 'Observer<Customer | null>'.
        Type 'Customer | null' is not assignable to type 'null'.
          Type 'Customer' is not assignable to type 'null'.ts(2322)

Customer Model

export class Customer {
    customerNo: string;
    customerAccountNo: string;
    customerName: string;
}