I placed the same custom function in a module in 2 workbooks, using
identical code, where it is the only contents of the VBA module. One works
fine, the other does not! Instead of displaying the resulting value I get
the text I typed in to call the function.

My function is AddCC(). The call AddCC("A,B","C") should show the result
"A/C,B/C".
In one workbook this is OK, in the other the cell displays
'AddCC("A,B","C")', with the formula showing as '=AddCC("A,B","C")'

Have I missed an option somewhere. Any other ideas, as I am beat!

CODE
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Option Explicit

Option Base 1 ' All arrays to start at 1, not 0

Function AddCC(szAccount As String, szCC As String)

Dim szNewString As String 'Work string to return

Dim i As Integer 'Counter

szAccount = Trim(szAccount)

'Return account + CC if there is only one

If InStr(szAccount, ",") = 0 Then

AddCC = szAccount & "/" & szCC

Exit Function

End If

For i = 1 To Len(szAccount)

If Mid(szAccount, i, 1) < "," Then

'Add next char if not a comma

szNewString = szNewString & Mid(szAccount, i, 1)

Else

'Char is a comma, so add CC

szNewString = szNewString & "/" & szCC & ","

End If

Next

If Right(szAccount, 1) < "," Then

szNewString = szNewString & "/" & szCC

End If

AddCC = szNewString

End Function