Delete Parameter Getting Set To NULL Why?

I have datagrid whose data source is set to SqlDataSource. SqlDataSource's connection string is set dynamically at pageload. When i click on delete link in gridview it executes delete command but row doesnt get deleted from data source. When i run Sql server profiler i found the delete statement is getting executed but the parameter is always null. why?

  <asp:GridView ID="GridView1" runat="server" AllowPaging="True" AllowSorting="True"
AutoGenerateColumns="False"
Caption="Events emitted by the application." DataKeyNames="EventId" OnDataBound="GridView1_DataBound" Width="600px" OnSelectedIndexChanged="GridView1_SelectedIndexChanged" OnRowDeleting="GridView1_RowDeleting" OnRowCommand="GridView1_RowCommand">
<Columns>
<asp:BoundField DataField="EventId" HeaderText="EventId" SortExpression="EventId" />
<asp:BoundField DataField="EventTime" HeaderText="EventTime" SortExpression="EventTime" />
<asp:BoundField DataField="Message" HeaderText="Message" SortExpression="Message" />
<asp:CommandField SelectText="Vew Details" ShowDeleteButton="True" ShowSelectButton="True" />
</Columns>
</asp:GridView>
<asp:SqlDataSource ID="SqlDataSource2" runat="server" DeleteCommand="DELETE FROM aspnet_WebEvent_Events WHERE (EventId = @EventId)"
SelectCommand="SELECT EventId,EventTime,Message FROM aspnet_WebEvent_Events WHERE (ApplicationVirtualPath = @ApplicationVirtualPath)">
<SelectParameters>
<asp:ControlParameter ControlID="DropDownList2" Name="ApplicationVirtualPath" PropertyName="SelectedValue" />
</SelectParameters>
<DeleteParameters>
<asp:Parameter Name="EventId" Type="String"/>
</DeleteParameters>
</asp:SqlDataSource>

 protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
}

protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "Delete")
SqlDataSource2.Delete();
}