Use SpyOn on provider that has string value

I have a class that I am trying to test. The class has a service Injected in as such:

 constructor(private actions$: Actions, @Inject('NotificationHandlerService') private notificationService: INotificationHandlerService) {}

In the spec.ts I just provided it using a value as I just want to test that the display function gets called:

  beforeEach(() => {
    TestBed.configureTestingModule({
      providers: [
        provideMockActions(() => actions$),
        RequirementNotificationsEffects,
        {
          provide: 'NotificationHandlerService',
          useValue: {
            display: () => {}
          }
        },
        provideMockStore({ initialState: requirementInitialState})
      ],
    });

    reqNotificationEffects = TestBed.inject(RequirementNotificationsEffects);
  });

I am then testing to see if the display function gets called however I receive an error on running the test stating Cannot spyOn on a primitive value; string given Test:

    it('Call notification service to display message to user.', () => {
      const testOwner = new OwnerGroupReference({name: 'Test Owner'}) ;
      const action = reqActions.removeOwnersWithSubmission({ owners: [{testOwner}]});
      const notificationDisplaySpy = jest.spyOn('NotificationHandlerService', 'display');

      actions$ = hot("-a", { a: action });

      expect(reqNotificationEffects.removeOwners$).toSatisfyOnFlush(() => {
        expect(notificationDisplaySpy).toHaveBeenCalled();
      });
    });

anyone know what I should be doing?

Extract useValue in a const variable

const mockService = {
  display: jest.fn();
}
beforeEach(() => {
  TestBed.configureTestingModule({
    providers: [
      provideMockActions(() => actions$),
      RequirementNotificationsEffects,
      {
        provide: 'NotificationHandlerService',
        useValue: mockService
      },
      provideMockStore({ initialState: requirementInitialState})
    ],
  });

  reqNotificationEffects = TestBed.inject(RequirementNotificationsEffects);
});

it('Call notification service to display message to user.', () => {
  const testOwner = new OwnerGroupReference({name: 'Test Owner'}) ;
  const action = reqActions.removeOwnersWithSubmission({ owners: [{testOwner}]});

  actions$ = hot("-a", { a: action });

  expect(reqNotificationEffects.removeOwners$).toSatisfyOnFlush(() => {
    expect(mockService.display).toHaveBeenCalled();
  });
});

You can implement this with spyOn also