JS error when adding grid columns dynamically

<zk xmlns:w="client">
    <button id="btn" label="Click me"/>
    <grid           
        sizedByContent="true"
        span="true">
        <columns fulfill="btn.onClick">
            <column 
                label="AAA">
            </column>
            <column 
                label="BBB"
                hflex="1">
            </column>
        </columns>
        <rows>
            <row>
                <label value="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" />
                <label value="bbbbbbbbbbbbbbbbb" />
            </row>
        </rows>
    </grid>
</zk>