[AccessD] vb.Net - Checking for NULL values

I am using Visual Studio 2005 Express.  I want to check whether a 
session value has been initialised and if not then set some 
values.  If there is already a value in Session("ItemAmt") then I 
don't want the values to change.  My code in the Page Load event is:

         If IsNull(Session("ItemAmt")) Then
             Session("ItemName") = "Description"
             Session("ItemAmt") = "100"
         End If

The problem is with the IsNull function.  I have not been able to 
find out the equivalent method in VB.net.  Any pointers?


Regards

David Emerson
Dalyn Software Ltd
New Zealand