Cannot read property 'has' of null

[MyTurnyet]

TypeError: Cannot read property 'has' of null
    at IndustryDataApiService.descriptor.value (D:\Development\Java\IndustryManager\node_modules\ngx-liquid-cache\bundles\ng:\ngx-liquid-cache\lib\services\liquid-cache.service.ts:165:58)
     at D:\Development\Java\IndustryManager\src\app\services\industry-data-service\industry-data.service.spec.ts:132:15

Trying to write tests with LiquidCache annotation on a function is returning this error. I may be missing something in my configuration, or mocking?

IndustryDataApiService looks like this:

 @Injectable()
 export class IndustryDataApiService extends IndustryDataService {


  constructor(
    private http: HttpProvider,
    private environmentConfig: EnvironmentConfigService,
    private cacheService: LiquidCacheService,
    private featureToggleService: FeatureToggleService,
  ) {
    super();
  }

 @LiquidCache('carTypes', {duration: 300})
  getAllCarTypes(): Observable<CarType[]> {

    if (!this.featureToggleService.isActive('retrieveCarTypes')) {
    const carTypesDataArray: CarType[] = [boxcar, flatcar];
     return this.getObservable<CarType[]>(carTypesDataArray);
    }
    const allCarTypes = this.http.getAll<CarType>(
      this.environmentConfig.carTypeGetAllApiUrl(),
    );

    return allCarTypes;
  }
}

Test looks like this:

describe('CarType Data', () => {
    const carTypesReturned: CarType[] = [boxcar, flatcar];

    it('should return getAllLocations data from API', (done) => {
      when(mockHttpProvider.getAll<CarType>(anyString())).thenReturn(of(carTypesReturned));

// ERROR OCCURS WHEN CALLING THE FOLLOWING LINE
      service.getAllCarTypes().subscribe((response: CarType[]) => {
        expect(response).toEqual(carTypesReturned);
        done();
      });
    });
});

I can get the code working if I pass in the LiquidCacheService and write the caching code manually, but I'd rather not. Any ideas?