Error :the cast to value type “Double” failed because the materialized value is null

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ASP.NET

MVC

MVC3

MVC2

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i have a star_tbl . i wanna make starpart in my web site. my cod:

public int StarProduct(int id_Product)
{
  return Convert.ToInt32(db
    .tbl_satar_Product
    .Where(p => p.Id_Product == id_Product && p.star != null)
    .Average(s => s.star));
}

but t shows this error :

the cast to value type "Double" failed because the materialized value is null. Either the result type 's generic parameter or the query must use a nullable type.


i used also (s=> s.star ?? 0d) but it says :
" operator '??' cannot be applied to operands of type 'double' and 'double' "