Whenever I enable default paging on my application I obtain DBNull errors

I am using ASP.NET with the VB.NET language. I am creating an online shopping mall with a database which contains some null values, a strongly typed dataset and a Business logic layer. A lot of my code is based on the asp.net data access tutorials.

Whenever I utilize default paging using the pageddatasource class in the business logic layer,  with sqldatasource I obtain DBNull errors.

For example I will use the following code in the .aspx page

<asp:TemplateField SortExpression="ProductName" >
        & ...