Nullable(Of Date) -- How to pass a NULL one?

I have a class that has 2 date fields.  BeginDate is always required, but EndDate is Nullable.

Public Class MyClass
        Private _BeginDate As Date
        Private _EndDate As Nullable(Of Date)

        Public Property BeginDate() As Date
            Get
                Return _BeginDate
            End Get
            Set(ByVal value As Date)
                _BeginDate = value
            End Set
        End Property

        Public Property EndDate() As Nullable(Of Date)
            Get
                Return _EndDate
            End Get
            Set(ByVal value As Nullable(Of Date))
                _EndDate = value
            End Set
        End Property
End Class

Now ... if a textbox is empty, I want to pass the NULL, not a date.  that would be why I used a NULLABLE ... so that I could have a NULL.

dim MyVar as new MyClass
MyVar.EndDate = IIf(tb_NewEffectiveEndDate.Text.Trim = "", System.DBNull.Value, tb_NewEffectiveEndDate.Text.Trim)

And I get an invalid cast exception when the box is empty.

Value of type 'System.DBNull' cannot be converted to 'System.Nullable(Of Date)'.

If I create a New Nullable(Of Date) it has a value already.  I can't create an empty / NULL one.  The HasValue property is ReadOnly ...

So ... how do a pass a NULL Nullable as opposed to a non-NULL nullable?  Or ... if I can't Null the Nullable is the Nullable really Nullable?  

Thanks!
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