Trouble creating custom d.ts for untyped NPM module

I've been stuck on this for a while now. I'm trying to use the npm package 'markerwithlabel', which is untyped. Here is the d.ts file I created

    declare module 'markerwithlabel' {
    export interface MarkerWithLabelOptions extends google.maps.MarkerOptions {
        labelContent?: string | Node;
        labelAnchor?: google.maps.Point;
        labelClass?: string;
        labelStyle?: Object;
        labelInBackground?: boolean;
        labelVisible?: boolean;
        raiseOnDrag?: boolean;
        optimized?: boolean;
        crossImage?: string;
        handCursor?: string;
    }

    export default class MarkerWithLabel extends google.maps.Marker {
        readonly prototype: any;
        constructor(opt_options?: MarkerWithLabelOptions);
    }
}

My implementing code

    import MarkerWithLabel from 'markerwithlabel';
...
    let marker = new MarkerWithLabel({ position: branchPosition, title: title, label: label, map: this._map });

Which is giving me the error

Uncaught TypeError: Cannot read property 'prototype' of undefined
    at inherits (index.js:45)
    at new module.exports (index.js:88)
    at eval (eval at GoogleMapsRouteRenderer.renderMarkersFromResult (google-maps-route-renderer.js:NaN), <anonymous>:1:1)
    at GoogleMapsRouteRenderer.renderMarkersFromResult (google-maps-route-renderer.js:139)
    at GoogleMapsRouteRenderer.render (google-maps-route-renderer.js:570)
    at MapSchedulerCustomElement.eval (map-scheduler.js:237)
    at step (map-scheduler.js:41)
    at Object.eval [as next] (map-scheduler.js:22)
    at fulfilled (map-scheduler.js:13)
    at tryCatcher (bluebird.js:4954)
    at Promise._settlePromiseFromHandler (bluebird.js:3095)
    at Promise._settlePromise (bluebird.js:3152)
    at Promise._settlePromise0 (bluebird.js:3197)
    at Promise._settlePromises (bluebird.js:3277)
    at _drainQueueStep (bluebird.js:190)
    at _drainQueue (bluebird.js:183)

Stepping into the constructor before getting the error I get to this line where OverlayView is undefined

inherits(MarkerLabel_, gMapsApi.OverlayView);

And lastly stepping into the inherits function where parentCtor.prototype is undefined

function inherits(childCtor, parentCtor) {
/** @constructor */
function tempCtor() {};
tempCtor.prototype = parentCtor.prototype;

Any help would be wonderful. Thanks.