"this.appInits[i]" is not a function

I am trying to use APP_INITIALIZER TO load data from config file. I'm getting following error:

"Unhandled Promise rejection: this.appInits[i] is not a function ; Zone: ; Task: Promise.then ; Value: TypeError: this.appInits[i] is not a function"

Code:

import { Injectable } from '@angular/core';
import { Http } from '@angular/http';
import { Observable } from 'rxjs/Rx';

@Injectable()
export class ApiConfig {

  private urlList: any = null;

  constructor(private http: Http) {

  }

  public getUrl(key: any) {
    return this.urlList[key];
  }

  public loadConfig(){

    let retPromise: Promise<any> =  this.http.get('./assets/config/api-config.json')
      .map(res => res.json()).toPromise();

    retPromise.then(resp => this.urlList=resp.urlList);
    //this.urlList = retPromise.urlList;

    return retPromise;
  }

}


export  function apiConfigProvider(config: ApiConfig) {
   config.loadConfig();
}

Any help on what I'm doing wrong?

Edit:

Adam's solution fixed the original error. But now I see a new error:

TypeError: Cannot set property 'urlList' of undefined

As I understand there is no instance of ApiConfig created. But all the examples I see in loading the config file in Angular give same example. I wonder how to force the creation of ApiConfig class. And how I can make it a singleton? Below is the usage of ApiConfig.

export BaseService { 
constructor (private config:ApiConfig){} 

public GetUrl (key: string) { 
   return config.urlList[key]; 
} 

}

Answer: It looks like there is an issue with the way promise is created in above code. I modified my loadconfig() method as given in the link: https://gist.github.com/fernandohu/122e88c3bcd210bbe41c608c36306db9 this resolved my problem.

I think this is because you need to return the function in your export function statement:

export function apiConfigProvider(config: ApiConfig) {
   return () => config.loadConfig();
}