Adding a CommandBarButton to the CommandBar

I am trying to create a COM Add-in for the MS Word and I wrote the following function of which I used to add a CommandBarButton to the "Web" CommandBar in the MS Word. When this function was called during the startup of MS Word in the OnStartupComplete(), the following error message displayed:

http://hk.geocities.com/saratara_me/errmsg.JPG

Could anybody please tell me what the problems could possibly be?

Many thanks to you all! :D

VB Code:

Public Function AddToCommandBar() As CommandBarButton
 
Dim cbb As CommandBarButton
Dim Cnt As Integer
 
Cnt = Application.CommandBars("Web").Controls.Count
 
Set cbb = Application.CommandBars("Web").Controls.Add(Type:=msoControlButton, Id:=2950, Before:=Cnt + 1)
 
Set AddToCommandBar = cbb
 
End Function