How to add null value to a foreign key column in SQL Server 2008?

Question

I am currently working on a Windows Forms application using C#. I have two tables (tblBatch & tblProduct). These tables are related to each other based on the BatchID.

i.e

-> tblBatch
    BatchID
    BatchName

And

-> tblProduct
    ProductID
    ProductName
    BatchID

Now there are two different situations.

1. Product registration (adding a specific item to the database - stalk quantity could be empty because this will just register a product.
2. Supplying stalk quantity, updating the quantity amount of the products and assigning the batchNo to products.

At this point, the batchID must already be present in the database before I would assign it to a product in tblProduct. Because, product batch cannot be assigned before supply of products, and as during this stage, I don't have any supply which can give me an idea of the current batchNo. So during product registration, I just want to assign null to the BatchID in tblProduct. Once a supply will be performed later, I will update this ID and will assign the exact BatchID to the supplied Items.

However, I am getting the following error even I have tried keeping the BatchID field null in tblProduct but still I am unable to add record into the database.

The INSERT statement conflicted with the FOREIGN KEY constraint "FK_tblProduct_tblBatch". The conflict occurred in database "myDB", table "dbo.tblBatch", column 'BatchID'.

Is there any solution to this problem? Or someone suggest me a simple way to resolve this specific problem.