Insert NULL Into Database If Form Field Is Empty Using ASP.NET & VB?

I have a form and stored procedure that inserts the data from the form. It works fine except that if a field isn't filled in it doesn't insert a NULL into SQL it inserts "".

I've tried a few different ways but none seem to insert NULL, the one below still inserts "", can anyone point me in the right direction?

Here is the required part of the code, if you require more just let me know.

Dim rdr As SqlDataReader
Dim cmdInsert As SqlCommand = New SqlCommand()
cmdInsert.CommandText = "spPersonalDetailsInsert"

[Code].....

So if I enter nothing into address1 field it should write NULL to screen but it always writes NOT NULL. What does an empty form field equal? in classic ASP it was always "".