Setting a checkbox as readonly

Hi,

I am trying to make a checkbox non-editable as below -

<jcr:rootxmlns:cq="http://www.day.com/jcr/cq/1.0"xmlns:jcr="http://www.jcp.org/jcr/1.0"xmlns:nt="http://www.jcp.org/jcr/nt/1.0"

    jcr:primaryType="cq:Widget"

    fieldLabel="Is Master?"

    name="./isMaster"

    readOnly="{Boolean}true"

    type="checkbox"

    xtype="selection">

However, the field is still editable.This works well for a text field, but doesnt in case of a checkbox.Any pointers??

Thanks for your help!