Converting feature from draw interaction to WKT string

Question

I've problems with converting the feature which I'm getting in draw interaction into the WKT string. I saw multiple examples which looks like it should be simple but I'm always getting the following exception while calling format.writeGeometry() function:

Here is my code:

function getDrawInteraction() {
    return new ol.interaction.Draw({
        source: this.source, //it is ol.source.Vector() for my layer
        stopClick: true,
        type: "Circle"
    });
}

function draw() {              
    var drawInteraction = getDrawInteraction(),
        format = new ol.format.WKT();

    drawInteraction.on("drawend", function (event) {
        var feature = event.feature,
            wkt;

        wkt = format.writeGeometry(feature.getGeometry());

    });
    map.addInteraction(drawInteraction);        
}

I'm able to draw a circle on the map but what I am doing wrong WKT string?