User Defined Function 'forgets' range, refresh problem

Hi,
the UDF SumFruits works fine in Sheet1. In another sheet in the same workbook the formula doesn't work properly. If the item in Sheet2 changes, '#Value' is returned. Sheet2 is only correctly recalculated, if Sheet1 changes. It seems that the function 'forget' the header range. Any ideas?

Thanks,
Georg


Data in Sheet1     (this is the header)
A      B
Product      Quantity
Apples      10
Oranges      12
Bananas      8
Pears      5
Apples      7
Oranges      15
Bananas      12



UDF
Option Explicit

Public Function SumFruits(Header As Range, Item As String) As Long
Dim rngCrit As Range                  'Criteria Range
Dim rngSum As Range                 'Range to summarize
Dim rngWA As Range                  'Range workaround
Dim shBW As Worksheet              'Sheet with the data
Dim c As Range
Dim intCritCol As Integer

   Application.Volatile

   'set the data sheet
   Set shBW = Header.Worksheet
   ' * * *  Does not work, Header.CurrentRegion returns not the whole range
'   'find Criteria Range for the SumIf formula
'   For Each c In Header.CurrentRegion
'      If c.Text = Item Then
'         intCritCol = c.Column
'      End If
'   Next c

   ' * * *  Workaround
  Set rngWA = shBW.Range("A4:B8")
   'find Criteria Range for the SumIf formula
   For Each c In rngWA
      If c.Text = Item Then
         intCritCol = c.Column
      End If
   Next c

   'set  Criteria and Summation Range
   Set rngCrit = shBW.Range(Cells(Header.Row, intCritCol), Cells(20000, intCritCol))
   Set rngSum = shBW.Range(Cells(Header.Row, Header.Column + 1), Cells(20000, Header.Column + 1))

   'sum the values
   SumFruits = Application.WorksheetFunction.SumIf(rngCrit, Item, rngSum)

End Function