Error 9 when launching add-in userform from Ribbon in Excel 2007

[Matt]

I spent the entire day trying to work around an issue I'm having with
launching an userform that is built into an add-in installed with its
own ribbon in Excel 2007. One of my buttons in the Ribbon calls a
simple subroutine in the Add-in as follows....

Sub SCAD()
Load SCADOptions: SCADOptions.Show
End Sub

When I click the button on my Add-in's ribbon in Excel 2007, I
immediatley get an "Error 9." After I click debug I get an 'out of
range error.'

Meanwhile, I can launch via Ribbon button any of my normal sub
routines out of the Add-in's modules. Is there some reason why I
cannot launch userforms via a sub routine from buttons in my custom
ribbon, but still I can launch any of my other subroutines from a
button on my custom ribbon?

Any help you can provide will be appreciated.

Thanks,
Matt