TypeScript TS7015 Element implicitly has an any type because index expression is not of type number

Question

Im getting this compilation error in my Angular 2 app:

TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.

The piece of code causing it is:

getApplicationCount(state:string) {
    return this.applicationsByState[state] ? this.applicationsByState[state].length : 0;
  }

This however doesn't cause this error:

getApplicationCount(state:str