DataSource Controls :: The Null Value Cannot Be Assigned To A Member With Type System.Decimal Which Is A Non-nullable Value Type

I am making a website to store invoices so i can mail them to clients. So i got a table for clients, invoices and one with invoice products/services. When an invoice has no products/services added to them and it returns null i get the error "the null value cannot be assigned to a member with type System.Decimal which is a non-nullable value type."

I just started with linq. Before i always used mysql. Mostly i just had to call data from 1 table so forgive me if my linq query isn't really correct.

Dim invoices = From i In db.invoices _
Group Join p In db.products On i.id Equals p.invoiceId Into invoiceProducts = Group _
Select New With {i.client.naam, i.titel, (From iP In invoiceProducts Select iP.prijs).Sum}

i show the client's name, the title of the invoice and the total amount the clients will need to pay for products and services.