Null Reference Exception when run under machine name

[Shannon]

I am having a very stange issue happen to my web application. I am
trying to develop a simple shopping cart. The code works perfectly
when I run the application under http://localhost/myapp. However, when
I try to run the same app under the machine name http://mymachine/myapp
I recieve a Null Reference Exception and the page crashes. I can't
figure out why this is happening. The Null Reference is referring to a
Session variable containing a dynamic DataTable which has the items the
user has chosen to purchase. The Store and the Shopping Cart are all
in 1 web form (only trying to sell t-shirts). Can anyone give me a
hint as to what may be happening. I am attaching the necessary code
snippets. Thanks in advance.

Dim objDT as DataTable = New DataTable
Dim objDR as DataRow
Private Sub Page_Load(ByVal sender as System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load

If Not Page.IsPostBack Then
makeCart()
End If

Function makeCart()
objDT = New System.Data.DataTable("Cart")
'create the columns to store the items in the shopping cart
Session("Cart") = objDT
End Function

Public Sub AddToCart(ByVal s As Object, ByVal e As EventArgs) 'this
handles the click event to add items to the cart.
objDT = Session("Cart")
For Each objDR In objDT.Rows 'this is the line which throws the
Null Reference error. After debugging I found that the Session("Cart")
= Nothing. As I mentioned above, this problem does not occur when run
under 'localhost'.
'do some stuff here
Next
End Sub