Angular 2, MVC in the same project - Error when putting '/' at the end of the url

Question

I'm getting an error because when i put / at the end of the URL the angular 2 service is looking for the current URL plus the MVC controller URL,

For example:

Without /: localhost/controller/action

With /: localhost/controller/controller/action

this is my code:

MVC Controller:

[HttpGet]
public async Task<ActionResult> GetItemNames()
{
    var response = await _client.GetAsync("invoiceitems");
    var items = new List<Item>();

    if (response.StatusCode == HttpStatusCode.Unauthorized)
    {
        return Json(new { success = false, errors = new string[]{"Please refresh the page or login."} }, JsonRequestBehavior.AllowGet);
        }

        if (response.IsSuccessStatusCode) {
            var result = response.Content.ReadAsStringAsync().Result;
            items = JsonConvert.DeserializeObject<List<Item>>(result);
        }

        var itemNames = items.Select(i => new {
            code = i.plCode,
            name = i.name,
            searchText = i.name.ToLower()
        }).ToList();

        return Json(new { success = true, data = itemNames }, JsonRequestBehavior.AllowGet);
    }
}

Angular 2 service:

private invoiceUrl = 'invoice/';

getItemNames(): Observable<any> {
    const itemsUrl = this.invoiceUrl + 'getitemnames';
    return this.http.get(itemsUrl)
        .map((res: Response) => res.json());
        // TODO: Catch errors.
}

TIA :)