How to add a blank field in a DataGridViewComboBoxColumn

Question

I have a DataGridViewComboBoxColumn that works and populates well. All I want to know is how to add a blank field. This way if the user inadvertently selects something they can select the blank row rather than pressing Ctrl-0.

In my code I tried adding Items.Add(DBNull.Value) but to no avail, as shown in my code. All it does is give me an error which is why it is rem'd out.

Any ideas would be greatly appreciated.

Private Sub FillCombobox(strHeaderText As String, strPropertyName As String, strTableName As String)
    Using cboBox As New DataGridViewComboBoxColumn
        With cboBox
            .HeaderText = strHeaderText
            .DataPropertyName = strPropertyName
            .DataSource = GetData("Select * From " & strTableName)
            .ValueMember = .DataPropertyName
           '.Items.Add(DBNull.Value)
        End With

        dgrMain.Columns.Add(cboBox)
    End Using
End Sub

EDIT

Private Function GetData(ByVal sqlCommand As String) As DataTable
    Dim da = New OleDbDataAdapter(sqlCommand, strConn)
    Dim con As New OleDbConnection(strConn)
    con.Open()
    Dim command As OleDbCommand = New OleDbCommand(sqlCommand, con)
    da.SelectCommand = command
    Dim dt As DataTable = New DataTable()
    da.Fill(dt)
    con.Close()
    Return dt
End Function