Invalid use of property - passing Userform input to procedure

Hi all,

I have a userform with the following code for a command button:

Private Sub CommandButtonOK_Click()
    Dim sFormFamilyName As String
    Dim sFormGiveName As String
    Dim sFormFederation As String
    Dim sFormConfederation As String

    sFormFamilyName = UCase(Me.TextBoxFamilyName.Value)
    sFormGivenName = Me.TextBoxGivenName.Value
    sFormFederation = Me.ComboBoxFederation.Value
    sFormConfederation = Me.ComboBoxConfederation.Value

    AddNationalReferee sFormFamilyName, sFormGivenName, sFormFederation, sFormConfederation

    Me.Tag = "OK"
    Me.Hide
End Sub

Now when I run the form and hit the OK button I get the error message "Invalid use or property". I've been searching and trying different things for 2 days. Just when I change the procedure name it runs once or twice and then goes back to the error message. It is probably a minor thing I'm overlooking, but I just don't know anymore at the moment. I hope someone can help.

The procedure I'm calling is located in a User Module and looks like this:

Public Sub AddNationalReferee(ByVal sFamilyName As String, ByVal sGivenName As String, ByVal sFederation As String, Optional ByVal sConfederation As String = "CEV")

    Dim sDisplayName As String
    Dim iEmptyRow As Integer
    Dim oCell As Range
    
    sFamilyName = UCase(sFamilyName)
    sDisplayName = sFamilyName & " " & sGivenName
    
    MsgBox sFamilyName & vbCrLf & sGivenName & vbCrLf & sDisplayName & vbCrLf & sCountry & vbCrLf & sConfederation

    'Add national ref to Referee_list
    
    'Find first empty row on Referee_list worksheet
    iEmptyRow = 0
    For Each oCell In Worksheets("Referee_list").Range("A:A").Cells
        If IsEmpty(oCell) Then
            iEmptyRow = oCell.Row
            Exit For
        End If
    Next

    If iEmptyRow > 0 Then
        'Add national ref to Referee_list
        Worksheets("Referee_list").Cells(EmptyRow, 1) = sDisplayName
        Worksheets("Referee_list").Cells(EmptyRow, 2) = sGivenName
        Worksheets("Referee_list").Cells(EmptyRow, 3) = sFamilyName
        Worksheets("Referee_list").Cells(EmptyRow, 5) = sFederation
        Worksheets("Referee_list").Cells(EmptyRow, 6) = sConfederation
        Worksheets("Referee_list").Cells(EmptyRow, 10) = "Nat."
        Worksheets("Referee_list").Cells(EmptyRow, 11) = LCase(sGivenName & Left(sFamilyName, 1))
        Worksheets("Referee_list").Cells(EmptyRow, 13).Formula = "=CONCATENATE(LEFT(B" & EmptyRow & ";1);LEFT(C" & EmptyRow & ";1))"
    Else
        MsgBox "No more room in the list to add a national referee to the list of referees"
    End If
    
    
    'Add national ref to control sheet
    'Find first empty row on Referee_list worksheet
    iEmptyRow = 0
    For Each oCell In Worksheets("Control sheet").Range("A10:A32").Cells
        If IsEmpty(oCell) Then
            iEmptyRow = oCell.Row
            Exit For
        End If
    Next
    
    If iEmptyRow > 0 Then
        'Add national ref to control sheet
        Worksheets("Control sheet").Cells(EmptyRow, 1) = sDisplayName
    Else
        MsgBox "No more room in the control sheet to add a national referee"
    End If
    
    'When this works, in reset workbook, remove all local refs

End Sub

Thanks for your time and support.

KR, Bas