Angular google-maps not displayed

I'm creating an app in which I have a carpooling module. Now I want to display a map on which you'll be able to see carpoolers that are close to you. However, the map won't show..

I'm using a directive from

http://nlaplante.github.io/angular-google-maps/

I've added the google-maps to my dependencies and the directive is replaced by a google map, but it doesn't show anything.

I've got this directive in my view:

<google-map center="center" 
        zoom="zoom" 
        markers="markers" 
        style="height: 400px; width: 100%; display: block;">
</google-map>

I have all the variables in my controller:

 $scope.center = {
        latitude: 45,
        longitude: -73
    };
    $scope.markers = [];
    $scope.zoom = 8;

When I open firebug, I get following error:

Error: [$compile:multidir] Multiple directives [googleMap, markers] asking for new/isolated scope on: <google-map class="ng-isolate-scope ng-scope" center="center" zoom="zoom" markers="markers" style="height: 400px; width: 100%; display: block;">

I've tried a lot of things and searched for solutions, but none fixed my problem. Is there anyone that can help?

Thanks in advance. HS.