asp.net - EF throwing NullReferenceException on SaveChanges() after updating record properties.. sometimes

My Simplified database:

   +----------+
   |Products  |
   +----------+
   |ProductID |

   |ProdName  |
   |Desc      |
   |BrandID   |
   |CategoryID|
   +----------+

   +----------+
   |Brands    |
   +----------+
   |BrandID   |

   |BrandName |
   |ImageID   |
   |Desc      |
   +----------+

   +----------+
   |Categories|
   +----------+
   |CategoryID|
   |CategName |

   |ImageID   |
   |Desc      |
   +----------+

   +----------+
   |Images    |
   +----------+
   |ImageID   |
   |Path      |
   +----------+


   +----------+
   |ImageLinks|
   +----------+
   |ImageID   |
   |ProductID |
   +----------+

(Note: A product can have multiple images, but a brand/category can have at most one)

In ASP.NET,

   using (DBEntities db = new DBEntities()) 
   {
         Product product = db.Products.FirstOrDefault(p => p.ProductID == 1);
         if (product != null) product.Desc = "any value";
         db.SaveChanges(); //works



         Image image = db.Images.FirstOrDefault(i => i.ImageID == 1);
         if (image != null) image.Path = "any value";
         db.SaveChanges(); //works

         Brand brand = db.Brands.FirstOrDefault(b => b.BrandID == 1);
         if (brand != null) brand.Desc = "the same value as the old description";
         db.SaveChanges();  //works

         Brand brand = db.Brands.FirstOrDefault(b => b.BrandID == 1);
         if (brand != null) brand.Desc = "some new description";

         db.SaveChanges();  //throws null reference exception

         Category categ = db.Categories.FirstOrDefault(c => c.CategoryID == 1);
         if (categ != null) categ.Desc = "the same value as the old description";
         db.SaveChanges();  //works

         Category categ = db.Categories.FirstOrDefault(c => c.CategoryID == 1);
         if (categ != null) categ.Desc = "some new description";
         db.SaveChanges();  //throws null reference exception
   }

This is very weird!

Here is the stack trace of the NullReferenceException being thrown by SaveChanges()

Object reference not set to an instance of an object.
   at System.Web.UI.ParseChildrenAttribute.GetHashCode()
   at System.Collections.Generic.ObjectEqualityComparer`1.GetHashCode(T obj)
   at System.Collections.Generic.HashSet`1.InternalGetHashCode(T item)

   at System.Collections.Generic.HashSet`1.AddIfNotPresent(T value)
   at System.Collections.Generic.HashSet`1.UnionWith(IEnumerable`1 other)
   at System.Collections.Generic.HashSet`1..ctor(IEnumerable`1 collection, IEqualityComparer`1 comparer)
   at System.Collections.Generic.HashSet`1..ctor(IEnumerable`1 collection)
   at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.GetAttributes(Type type)
   at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.GetAttributes(PropertyInfo propertyInfo)
   at System.Data.Entity.Internal.Validation.EntityValidatorBuilder.BuildPropertyValidator(PropertyInfo clrProperty)
   at System.Data.Entity.Internal.Validation.EntityValidatorBuilder.BuildValidatorsForProperties(IEnumerable`1 clrProperties, IEnumerable`1 edmProperties, IEnumerable`1 navigationProperties)
   at System.Data.Entity.Internal.Validation.EntityValidatorBuilder.BuildTypeValidator[T](Type clrType, IEnumerable`1 edmProperties, IEnumerable`1 navigationProperties, Func`3 validatorFactoryFunc)
   at System.Data.Entity.Internal.Validation.EntityValidatorBuilder.BuildEntityValidator(InternalEntityEntry entityEntry)

   at System.Data.Entity.Internal.Validation.ValidationProvider.GetEntityValidator(InternalEntityEntry entityEntry)
   at System.Data.Entity.Internal.InternalEntityEntry.GetValidationResult(IDictionary`2 items)
   at System.Data.Entity.DbContext.ValidateEntity(DbEntityEntry entityEntry, IDictionary`2 items)
   at System.Data.Entity.DbContext.GetValidationErrors()
   at System.Data.Entity.Internal.InternalContext.SaveChanges()
   at System.Data.Entity.Internal.LazyInternalContext.SaveChanges()
   at System.Data.Entity.DbContext.SaveChanges()
   at VatechWebsite.Admin.Image_Upload.UploadBtn_Click(Object sender, EventArgs e) in c:\Users\Toshiba User\Desktop\vatech\VatechWebsite\VatechWebsite\Admin\Image_Upload.aspx.cs:line 109

PS: this is entity framework 5

Answer

As divega pointed out, the problem was that I had a class with the same name as the entity generated by EF (both for Category and Brand). Refer to this post NullReferenceException in DbContext.saveChanges()