Problem with adding new object to data base - asp.net

Question

Hi i have an error when i trying add object to database. Error message is:
Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=472540 for information on understanding and handling optimistic concurrency exceptions.
My adding methods:
public void AddProduct(string category, FormCollection formCollection, HttpPostedFileBase image) //Dodaje nowy produkt do bazy.
{
var product = GetNewProduct(category);
var EfContext = GetEfContext(category);
foreach(var property in product.GetType().GetProperties())
{
if(property.Name != "ImageData" && property.Name != "ImageMimeType")
{
var NewValue = Convert.ChangeType(formCollection[property.Name], property.PropertyType);
property.SetValue(product, NewValue);
}
else
{
if (property.Name == "ImageData")
{
property.SetValue(product, new byte[image.ContentLength]);
}
if(property.Name == "ImageMimeType")
{
property.SetValue(product, image.ContentType);
}
if(product.GetType().GetProperty("ImageData").GetValue(product) != null && product.GetType().GetProperty("ImageMimeType").GetValue(product) != null)
{
image.InputStream.Read((byte[])product.GetType().GetProperty("ImageData").GetValue(product), 0, image.ContentLength);
}
}
}
EfContext.GetType().GetMethod("AddProduct").Invoke(EfContext, new object[] { product });
}
And
public void AddProduct(GPU product)
{
product.Product_ID = productContext.items.Count() != 0 ? productContext.items.OrderByDescending(x => x.ProductID).Select(x => x.ProductID).FirstOrDefault() + 1 : 1;
context.GPUs.Add(product);
context.SaveChanges();
}