Datagridview MaskedTextBox Can't Remove Value

I have a winforms app with SQL for the database.

I have added a masked textbox to my datagridview using 00/00/0000 as the mask. When I try to remove the date from the cell I get the following error when I try to leave the cell.

I have used the following code to add the datagridview masked textbox to my winform app. Three classes MaskedEditColumn, MaskedEditCell, MaskedEditingControl. This code is far beyond my skills and I cannot figure out what I need to change to so the user can remove a date from the masked datagridview cell and have the cell empty.

The masked textbox is a nice way to make sure data is entered in the proper format.  Here is a way to place a maskedtextbox in the datagridview.

Public Class MaskedEditColumn
    Inherits DataGridViewColumn
    Public Sub New()
        MyBase.New(New MaskedEditCell())
    End Sub

    Public Overrides Property CellTemplate() As DataGridViewCell
        Get
            Return MyBase.CellTemplate
        End Get
        Set(ByVal value As DataGridViewCell)

            ' Ensure that the cell used for the template is a CalendarCell.
            If Not (value Is Nothing) AndAlso _
                Not value.GetType().IsAssignableFrom(GetType(MaskedEditCell)) _
                Then
                Throw New InvalidCastException("Must be a MaskedEditCell")
            End If
            MyBase.CellTemplate = value
        End Set
    End Property

    Private m_strMask As String
    Public Property Mask() As String
        Get
            Return m_strMask
        End Get
        Set(ByVal value As String)
            m_strMask = value
        End Set
    End Property

    Private m_tyValidatingType As Type
    Public Property ValidatingType() As Type
        Get
            Return m_tyValidatingType
        End Get
        Set(ByVal value As Type)
            m_tyValidatingType = value
        End Set
    End Property

    Private m_cPromptChar As Char = "_"c
    Public Property PromptChar() As Char
        Get
            Return m_cPromptChar
        End Get
        Set(ByVal value As Char)
            m_cPromptChar = value
        End Set
    End Property

    Private ReadOnly Property MaskedEditCellTemplate() As MaskedEditCell
        Get
            Return TryCast(Me.CellTemplate, MaskedEditCell)
        End Get
    End Property
End Class

Public Class MaskedEditCell
    Inherits DataGridViewTextBoxCell
    Public Overrides Sub InitializeEditingControl(ByVal rowIndex As Integer, _
        ByVal initialFormattedValue As Object, _
        ByVal dataGridViewCellStyle As DataGridViewCellStyle)
        ' Set the value of the editing control to the current cell value.
        MyBase.InitializeEditingControl(rowIndex, initialFormattedValue, _
            dataGridViewCellStyle)
        Dim mecol As MaskedEditColumn = DirectCast(OwningColumn, MaskedEditColumn)
        Dim ctl As MaskedEditingControl = _
            CType(DataGridView.EditingControl, MaskedEditingControl)
        Try
            ctl.Text = Me.Value.ToString
        Catch
            ctl.Text = ""
        End Try
        ctl.Mask = mecol.Mask
        ctl.PromptChar = mecol.PromptChar
        ctl.ValidatingType = mecol.ValidatingType
    End Sub

    Public Overrides ReadOnly Property EditType() As Type
        Get
            ' Return the type of the editing contol that CalendarCell uses.
            Return GetType(MaskedEditingControl)
        End Get
    End Property

    Public Overrides ReadOnly Property ValueType() As Type
        Get
            ' Return the type of the value that CalendarCell contains.
            Return GetType(String)
        End Get
    End Property

    Public Overrides ReadOnly Property DefaultNewRowValue() As Object
        Get
            ' Use the current date and time as the default value.
            Return ""
        End Get
    End Property

    Protected Overrides Sub Paint(ByVal graphics As System.Drawing.Graphics, ByVal clipBounds As System.Drawing.Rectangle, ByVal cellBounds As System.Drawing.Rectangle, ByVal rowIndex As Integer, ByVal cellState As System.Windows.Forms.DataGridViewElementStates, ByVal value As Object, ByVal formattedValue As Object, ByVal errorText As String, ByVal cellStyle As System.Windows.Forms.DataGridViewCellStyle, ByVal advancedBorderStyle As System.Windows.Forms.DataGridViewAdvancedBorderStyle, ByVal paintParts As System.Windows.Forms.DataGridViewPaintParts)
        MyBase.Paint(graphics, clipBounds, cellBounds, rowIndex, cellState, value, formattedValue, errorText, cellStyle, advancedBorderStyle, paintParts)
    End Sub
End Class

Class MaskedEditingControl
    Inherits MaskedTextBox
    Implements IDataGridViewEditingControl
    Private dataGridViewControl As DataGridView
    Private valueIsChanged As Boolean = False
    Private rowIndexNum As Integer
    Public Property EditingControlFormattedValue() As Object _
        Implements IDataGridViewEditingControl.EditingControlFormattedValue
        Get
            Return Me.Text
        End Get
        Set(ByVal value As Object)
            Me.Text = value.ToString
        End Set
    End Property

    Public Function EditingControlWantsInputKey(ByVal key As Keys, _
           ByVal dataGridViewWantsInputKey As Boolean) As Boolean _
           Implements IDataGridViewEditingControl.EditingControlWantsInputKey
        Return True
    End Function

    Public Function GetEditingControlFormattedValue(ByVal context _
        As DataGridViewDataErrorContexts) As Object _
        Implements IDataGridViewEditingControl.GetEditingControlFormattedValue
        Return Me.Text
    End Function

    Public Sub ApplyCellStyleToEditingControl(ByVal dataGridViewCellStyle As  _
        DataGridViewCellStyle) _
        Implements IDataGridViewEditingControl.ApplyCellStyleToEditingControl

        Me.Font = dataGridViewCellStyle.Font
        Me.ForeColor = dataGridViewCellStyle.ForeColor
        Me.BackColor = dataGridViewCellStyle.BackColor
    End Sub

    Public Property EditingControlRowIndex() As Integer _
        Implements IDataGridViewEditingControl.EditingControlRowIndex
        Get
            Return rowIndexNum
        End Get
        Set(ByVal value As Integer)
            rowIndexNum = value
        End Set
    End Property

    Public Sub PrepareEditingControlForEdit(ByVal selectAll As Boolean) _
        Implements IDataGridViewEditingControl.PrepareEditingControlForEdit
        ' No preparation needs to be done.
    End Sub

    Public ReadOnly Property RepositionEditingControlOnValueChange() _
        As Boolean Implements _
        IDataGridViewEditingControl.RepositionEditingControlOnValueChange
        Get
            Return False
        End Get
    End Property

    Public Property EditingControlDataGridView() As DataGridView _
        Implements IDataGridViewEditingControl.EditingControlDataGridView
        Get
            Return dataGridViewControl
        End Get
        Set(ByVal value As DataGridView)
            dataGridViewControl = value
        End Set
    End Property

    Public Property EditingControlValueChanged() As Boolean _
        Implements IDataGridViewEditingControl.EditingControlValueChanged
        Get
            Return valueIsChanged
        End Get
        Set(ByVal value As Boolean)
            valueIsChanged = value
        End Set
    End Property
    Public ReadOnly Property EditingControlCursor() As Cursor _
        Implements IDataGridViewEditingControl.EditingPanelCursor
        Get
            Return MyBase.Cursor
        End Get
    End Property

    Protected Overrides Sub OnTextChanged(ByVal e As System.EventArgs)
        ' Notify the DataGridView that the contents of the cell have changed.
        valueIsChanged = True
        Me.EditingControlDataGridView.NotifyCurrentCellDirty(True)
        MyBase.OnTextChanged(e)
    End Sub
End Class

I realize that the slashes in my mask are making the system believe there is data in the field but I'm not sure how to override this value to tell it the cell is empty.  I have tried putting code on the CellLeave and the CellValidating events and nothing seems to work.  It could be because I'm not exactly sure how to override that mask though.  This has been driving me nuts and if you understand how to fix my problem your help would be much appreciated.

Thank you,
Stacy