linq to sql in n-tier application

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VB

Hi every body, i am beginer in both linq to sql and n-tier applications, i made a project using Northwind database in vb.net, i have a problem with insert operation about null value error message, i made a sub to insert values in customers table it is as follows

public sub inscust(ByVal u as customer)

dim db as NorthWindDataContext=new NorthWindDataContext()

db.customers.insertonsubmit(u)

db.submitchanges()
end sub

this sub is in DataEntityTier ( intermediate layer )
in the presentation tier i called that sub as follows

Dim t as DataEntityTier.customer= new DataEntityTier.customer

Dim db as DataEntityTier.NorthWindDataContext= new NorthWindDataContext()

t.CustomerID= customersDataGridView.Rows(0).Cells(0).Value.ToString()
t.ComPanyName= customersDataGridView.Rows(0).Cells(1).Value.ToString

and i have made all columns then i called th inscust sub
Dim a As DataEntityTier.Class1 = new DataEntityTier.Class1()

a.inscust(t)
when running the application an bug and error message appear about null value
i want the insert operation allows null value
when i run i fill all columns of the datagridview . thanks dears