Forms Data Controls :: Handling Dbnull And Datetime?

I'm building an application that uses GridView controls to display data to a page.In my editemplate i have following control

<div class = "divBox">
<div>Beginn/Zeit 2</div>
<div>
<asp:TextBox ID="tbBeginn2" CssClass="fSelect"
runat="server" Text='<%# Bind("beginnDatum2") %>'/>
BeginnDatum2 has datetime-Format in SQL-Server.

For the update i use an objectDatasource with following updateparameter
<asp:Parameter Name="beginnDatum2" Type="datetime" />

Now i'm trying to update this field but i have a problem when BeginnDatum2 must be null.

I get following (german) Errormessage:

SqlDateTime-Überlauf; muss zwischen 1/1/1753 12:00:00 AM und 12/31/9999 11:59:59 PM liegen.
And this is my paramter
Public Function UpdateSeminar(ByVal kursid As Integer, ByVal id As Integer, _
ByVal beginnDatum2 As DateTime, _
ByVal endeDatum2 As DateTime, _
cmd.Parameters.Add(New SqlParameter("@beginnDatum2", SqlDbType.DateTime))
If beginnDatum2 = Nothing Then
cmd.Parameters("@beginnDatum2").Value = DBNull.Value
Else
cmd.Parameters("@beginnDatum2").Value = DateTime.Parse(beginnDatum2)
End If