Mil-tek Balers and Compactors

System.NullReferenceException: Object reference not set to an instance of an object.
   at Dynamicweb.eCommerce.Products.ProductGroupRelation.GetCachedProductGroupRelations(String productId)
   at Dynamicweb.eCommerce.Products.Product.get_PrimaryGroupID()
   at Dynamicweb.eCommerce.Products.Product.GetCategoryValue(String categoryID, String fieldID, Boolean includeInheritance)
   at Dynamicweb.eCommerce.Frontend.Renderer.GetProductCategoryFieldValue(Product product, String categoryID, Field field)
   at Dynamicweb.eCommerce.Frontend.Renderer.RenderProductCategories(Product product, Template template, String loopName)
   at Dynamicweb.eCommerce.Frontend.Renderer.RenderProduct(Product Product, Boolean ExtendedProperties, Template Template, Int32 loopCounter, Boolean renderRelated, String productURL, Int32 quantity, Int32 wishListID, Boolean renderPublicList, Int32 orderLineOfProductCount)
   at Dynamicweb.eCommerce.Frontend.Renderer.RenderProducts(ProductCollection Products, Template ParentTemplate, String LoopName, Int32 LoopCounter, Boolean renderRelatedProducts, Boolean renderRelatedLists)
   at Dynamicweb.eCommerce.Frontend.Renderer.RenderProducts(ProductCollection Products, Template ParentTemplate)
   at Dynamicweb.eCommerce.Frontend.Renderer.internalRenderProducts(ProductCollection products, Boolean isProductsFromIndex)
   at Dynamicweb.eCommerce.Frontend.Frontend.ListProducts(ProductCollection products, String groupsIdent, Boolean hasBeenSorted)
   at Dynamicweb.eCommerce.Frontend.Frontend.List(String groups)
   at Dynamicweb.eCommerce.Frontend.Frontend.EcomRender()
   at Dynamicweb.eCommerce.Frontend.Frontend.GetContent()
   at Dynamicweb.Frontend.Content.GetModule(DataRow& ParagraphRow)