Updating the Ribbon

My fluent code starts:

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="RibbonLoader">

And I have a checkbox called "csmFLC"

There is a global variable declared in a standard module:

Public g_RibbonUI As IRibbonUI

And the following routine:

Public Sub RibbonLoader(Thisribbon As IRibbonUI)
    Set g_RibbonUI = Thisribbon
End Sub

However any code using g_RibbonUI such as:

g_RibbonUI.InvalidateControl "csmFLC"

or

g_RibbonUI.Invalidate

results in a error message

Error 91: Object variable or With block variable not set.

It looks to me that g_RibbonUI should have been set and it is definitely the case that the RibbonLoader subroutine IS called. So any ideas as to what is going wrong?