Knockout binding css class to an observed model property

I want to bind a divs css class to a property of the view model like this:

<div id="statusIndicator" data-bind="css: selectedPriority">

But this generates the result:

 <div id="statusIndicator" class=" 0 1 2 3">

This is the view model:

myViewModel = {
    selectedPriority: ko.observable('High'),
    Company: ko.observable("Bert"),
    Rows: ko.observableArray([
         new row(),
         new row(),
         new row()
    ]),
    Tabs: ['High', 'Medium', 'Low'],

    selectPriority: function (tab) {
        this.selectedPriority(tab);
    }
};

So when i load the page that uses this view model i want the div to be:

<div id="statusIndicator" class="High">

What am i doing wrong?