Что означает ошибка An exception of type 'System.Data.Entity.Core.EntityCommandExecutionException' occurred in...?

JavaScriptSerializer json_serializer = new JavaScriptSerializer();
            List<string> returnMods = new List<string>();
            if (userRoles.Count != 0)
            {
                foreach (var role in userRoles)
                {
                    accessMod = db.AccessMods.First(n => n.RoleId == role.RoleId);
                    accessModString = (List<AccessModString>)json_serializer.DeserializeObject(accessMod.AccessString);
                    foreach (var item in accessModString) 
                    {
                        if (item.Name == nameMod)
                        {
                            returnMods.Add(item.Mod);
                        }
                    }
                }
                return Ok(returnMods);
            }

accessMod = db.AccessMods.First(n => n.RoleId == role.RoleId);