Cluster icon on click zooms out, Internet Explorer only

[motiejuss]

Case: http://kort.danskelbilalliance.web01.cabana.dk/

MarkerClusterer initialize code:

_markerClusterer = new MarkerClusterer(map, markers, {
    maxZoom: 15,
    gridSize: 60,
    styles: [{
        url: '/resources/img/DanskElbilAlliance/m1.png',
        height: 53,
        width: 52,
        //anchor: [16, 0],
        textColor: '#ffffff',
        textSize: 10
    }, {
        url: '/resources/img/DanskElbilAlliance/m2.png',
        height: 56,
        width: 55,
        //anchor: [24, 0],
        textColor: '#000000',
        textSize: 11
    }, {
        url: '/resources/img/DanskElbilAlliance/m3.png',
        height: 66,
        width: 65,
        //anchor: [32, 0],
        textColor: '#ffffff',
        textSize: 12
    }, {
        url: '/resources/img/DanskElbilAlliance/m4.png',
        height: 78,
        width: 77,
        //anchor: [32, 0],
        textColor: '#ffffff',
        textSize: 13
    }, {
        url: '/resources/img/DanskElbilAlliance/m5.png',
        height: 90,
        width: 89,
        //anchor: [32, 0],
        textColor: '#ffffff',
        textSize: 14
    }]
});

Anyone has an idea what's going on?