Datagridviewcombobox cell value is not valid

Hello i have a datagridview with one bound combobox added by coding in form load and all other columns are unbound and created at design time. When i run the project, type in Unitprice column and then move to the next column by pressing tab key the "datagridviewcomboboxcell value is not valid" error appears. My coding  of the project is here:

Private Sub CustomerBillForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        Dim ProductCombo As New DataGridViewComboBoxColumn()
        With ProductCombo
            .HeaderText = "ProductID"
            .Name = "ProductCombo"
            .MaxDropDownItems = 4
            .DataSource = ProductDataTable
            .DisplayMember = "ProductName"
            .ValueMember = "Productid"
            .DisplayIndex = 0
            .Width = 200
            '.Items.Add("True")
            '.Items.Add("False")
            '.ValueType = TypeOf(Decimal)
            BillGrid.Columns.Add(ProductCombo)

        End With
end sub

Private Sub BillGrid_CellLeave(sender As Object, e As DataGridViewCellEventArgs) Handles BillGrid.CellLeave

        Dim RowID As Integer = BillGrid.CurrentRow.Index
        Dim ColID As Integer = BillGrid.CurrentCell.ColumnIndex
        'BillGrid.Item(0, 0) = Convert.ToString(BillGrid.Columns("ProductName").v
        If BillGrid.Item(ColID, RowID).ColumnIndex = 1 Or 2 Or 3 Then
            BillGrid.Item(4, RowID).Value = (BillGrid.Item(1, RowID).Value - BillGrid.Item(3, RowID).Value) * BillGrid.Item(2, RowID).Value
        End If
    End Sub

please help .. i wasted hours on this ....