Passing tag from Office2010 ribbon

Hi,

I'm trying to assign a macro to a customised ribbon. I can do this ok, but I can't get it to pass a tag parameter at all.

My ribbon xml contains:

<mso:button idQ="x1:Letter_0_E3B85FB" label="Letter" imageMso="FileNew" onAction="Letter_2010" tag="letter" visible="true" />

and the macros is defined as:

Sub letter_2010(control As IRibbonControl)
MsgBox control.Tag
End Sub

However, when I click the button, I get an 'Argument not optional' error. If I make the sub definition ...(Optional control as IRibbonControl) and set a breakpoint on the MsgBox line, hovering over the 'control' object yields "control = Nothing".

I guess that the ribbon isn't passing the tag option, and I need to enable it somewhere. As much as I've searched I can't find the answer though!

Any help much appreciated!

Thanks,

Iain