VSTO 3 RibbonX getLabel Callback Failing

It seems I can not properly implement the callback for getLabel in VSTO 3 开发者_运维技巧with XML defined RibbonX. What am I doing wrong here? Is there some holy grail of documentation that I am missing out on?

public string GetCustomLabel(IRibbonControl control) {

    return "Custom label";

    }

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As it turns out label can not be assigned before getLabel in the XML.