Debugging vsto ribbon callbacks - vsto

Question

I have a VSTO Outlook ribbon (context menu), which works as expected:
<?xml version="1.0" encoding="UTF-8"?>
<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="Ribbon_Load">
<contextMenus>
<contextMenu idMso="ContextMenuMailItem">
<button id="DoThis"
label="Label"
onAction="DoThis"
getVisible="GetVisible"/>
</contextMenu>
</customUI>
However, if I have a getLabel attribute, then the context menu does not show up at all anymore. I guess I must be screwing something up, but there is no indication what; no log, no exception, nothing. Furthermore, I can't find anywhere that documents what the definition of each callback should be. I just tried the obvious, that getLabel should return a string, but it does not seem to work. getVisible works just fine (returning a bool).
<?xml version="1.0" encoding="UTF-8"?>
<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="Ribbon_Load">
<contextMenus>
<contextMenu idMso="ContextMenuMailItem">
<button id="DoThis"
label="Label"
onAction="DoThis"
getVisible="GetVisible"
getLabel="GetLabelMoveToReviewFolderMultiple"/>
</contextMenu>
</customUI>
and the code behind (other methods not shown):
[ComVisible(true)]
public class ContextMenu : Office.IRibbonExtensibility
{
public string GetLabelMoveToReviewFolderMultiple(Office.IRibbonControl control)
{
return "Custom Label";
}
}

Answer 1

Do not use both label and getLabel. Also, enable addin errors in File | Options | Advanced | Developer to see all ribbon XML errors.

Answer 2

Well, while writing the question, I tried removing the label attribute for the context menu using getLabel, and that does resolve the issue; context menu works fine after that. I (initially) thought it might make sense to keep label as a default.
The documentation explains what is mutually exclusive.