Angular2 Component Unit Testing with Service Dependency - unit-testing

Question

So I have this PromptComponent and the content will be updated by a service data.
Now I am trying to do the unit test this component which has this service dependency.
Here is the Component:
export class PromptComponent {
#Input() module: any;
#select() selectedPrompt;
constructor(private moduleService: ModuleService){}
}
Here is the service:
getQuote(): Promise<string> {
return new Promise(resolve => {
setTimeout( () => resolve(this.nextQuote()), 500 );
});
}
private nextQuote() {
if (this.next === quotes.length) { this.next = 0; }
return quotes[ this.next++ ];
}
Here is the unit test code:
beforeEach(() => {
TestBed.configureTestingModule({
declarations: [
PromptComponent,
],
providers: [ ModuleService]
});
fixture = TestBed.createComponent(PromptComponent);
component = fixture.componentInstance;
const moduleService = fixture.debugElement.injector.get(ModuleService);
spy = spyOn(moduleService, 'getQuote').and.returnValue(Promise.resolve(testService));
I referred to the official demo on the angular documentation
https://angular.io/resources/live-examples/testing/ts/eplnkr.html, as inside the TwainComponent.spec.
Something really weird is that as expected, inside the official demo, the TwainComponent test passed smoothly. But I, on the other hand, got this strange injectionError. If I remove the parameter inside the constructor, I am able to create the fixture instance but get injection error when it runs thorught the injector.get() part. But as soon as I put the private moduleService:ModuleService back inside the component constructor, I will get fixture undefined and also with the injection error.
So that means I cannot even execute my code to the actual service injection part (fixture.debugElement.injector.get(ModuleService)), I already got the error.
Does anybody have the same issue?
My code is almost identical to the demo code on Plunker. Have no clue how to move forward....