I have this function:

Function hoogste(Optional maxweek As String) As String

Dim sh As Worksheet, sName As String
Dim maxindex As Long

maxweek = 0
maxindex = 0
For Each sh In ActiveWorkbook.Sheets
sName = sh.Name
If IsNumeric(sName) Then
If CLng(sName) < 53 Then
If CLng(sName) maxweek Then
'maxweek = CLng(sName)
maxweek = sName
maxindex = sh.Index
End If
End If
End If
Next
hoogste = "1:" & maxweek
End Function

It returns 1:12 if my highest sheetname is 12.

Now i want to sum cel c10 of all sheets 1 to 12

=SUM('1:12'!C6)
Every time my highest number is an other so i would like to use
=SUM('hoogste()'!C6)

this does not work, what am i doing wrong?

Greetings Hans

PS i use excel 2003