I'm still learning about scope and visibility of objects, but I thought I had
a general handle on the concept. This problem, though, has me stonkered:

In a sheet module, declarations section:
Public wbo

The same sheet module, the first procedure, the first two statements:
Set wbo = Workbooks("MyWorkbook.xls")
Stns = RangeValues("Map", 2, 4)

RangeValues is in a separate module named "Common". It starts out like this:
Function RangeValues(SheetName, RowA, ColZ)
Set wso = wbo.Worksheets(SheetName)

So we enter RangeValues with the first argument set to "Map", and when it
gets to the statement "Set wso=wbo.Worksheets(Sheet)" I expect it to set wso
to the worksheet named "Map". Instead I get an error 91:

Run-time error '91':
Object variable or With block variable not set.

This SEEMS to say that wbo isn't properly initialized as a workbook object.
Yet just before it executes the Set statement I can see wbo in the Watch
window, with the Map sheet as one of its properties. So is the error message
talking about some other object? Can anyone see the obvious piece I'm
missing?