XLA not loading properly on start-up
 

I have an XLA with User Defined Functions in it. Upon loading I attempt to
load one of the function into a function category in Excel (UDF category 14)
When Excel starts up and opens the XLA I get an error message in the
ThisWorkbook module:

"Run time Error 1004: Cannot edit a macro on a hidden workbook. Unhide the
workbook using the Unhide command"

The code is as follws:
Private Sub Workbook_Open()
Application.MacroOptions Macro:="VowelCount", Category:=14
End Sub

Why is this happening? The XLA is obviously hidden. Do I have to
momentarily unihde it to add the funciton and then hide it again?

Thanks