Love the simplicity of lib but im unable to use it in component testing with angular TestBed.

Test:

describe('FilterContentWidgetComponent', () => {
    let component: FilterContentWidgetComponent;
    let fixture: ComponentFixture<FilterContentWidgetComponent>;

    let filterMapperMock = Substitute.for<FilterMapper>();
    
    beforeEach(async() => {
        TestBed.configureTestingModule( {
            declarations: [ 
                FilterContentWidgetComponent,
            ],
            imports: [ HttpModule], 
            providers: [ 
                { provide: FilterMapper, useValue: <FilterMapper>filterMapperMock},
                
             ]
        }).compileComponents();
    });

    beforeEach(() => {
        fixture = TestBed.createComponent(FilterContentWidgetComponent);
        component = fixture.componentInstance;
      });

    it('should create', () => {
        fixture.detectChanges();

        expect(component).toBeTruthy();
    });
});

Not sure if am doing sth wrong but i get following exception.

TypeError: Cannot convert object to primitive value at JitEmitterVisitor._emitReferenceToExternal (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:4693:44) at JitEmitterVisitor.visitExternalExpr (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:4662:1) at ExternalExpr.visitExpression (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:1293:1) at visitAllObjects.expr (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:4387:1) at JitEmitterVisitor.visitAllObjects (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:4406:1) at JitEmitterVisitor.visitAllExpressions (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:4387:1) at JitEmitterVisitor.visitInvokeFunctionExpr (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:4218:1) at JitEmitterVisitor.visitInvokeFunctionExpr (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:4547:1) at InvokeFunctionExpr.visitExpression (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:1250:1) at visitAllObjects.expr (http://localhost:9876/node_modules/@angular/compiler/fesm2015/compiler.js?:4387:1)