Custom Grouping Problem

I am having some trouble with grouping. I have a custom function that returns the field to group on depending on the parameters provided by the user. This is not working, however if I select the same value from the grouping drop down list that is returned by the custom function the grouping works. My custom funtion is as follows:

Function OrganisationGroup() As String
Dim Group As String
Group = ""
Select Case Report.Parameters!EnterpriseOption.Value
Case "1"
Group = Group1Value()
Case "2"
Group = "DepartmentDesc"
Case "3"
Group = "OrganizationDesc"
Case "4"
Group = "OrganizationDesc"
End Select

If Group = "" Then
Return "0"
Else
Return "Fields!" & Group & ".Value"
End If
End Function

Function Group1Value() As String
Dim Group As String
Group = ""
Select Case Report.Parameters!ReportType.Value
Case "1"
Group = "StockFamilyDesc"
Case "2"
Group = "GroupDesc"
Case "3"
Group = "Saleable"
Case "4"
Group = "Saleable"
Case "5"
Group = "SaleableGLCodeDesc"
Case "6"
Group = "Saleable"
End Select
Return Group
End Function

Does anyone have any ideas on why the grouping would not be working with the custom function?

Thanks in advance.