How can I check for a null value while loading a list with a reader??

public static List<Subcategories> GetSubcategories(int SubgroupID)

using (SqlConnection connection = new SqlConnection(ConfigurationManager.ConnectionStrings["XXX"].ConnectionString))

using (SqlCommand command = new SqlCommand("GetSubcategories", connection))

command.CommandType = CommandType.StoredProcedure;

command.Parameters.Add(new SqlParameter("@SubgroupID", SubgroupID));

List<Subcategories> list = new List<Subcategories>();

using (SqlDataReader reader = command.ExecuteReader())

while (reader.Read())

Subcategories temp = new Subcategories(

(int)reader["SubCategoryId"],

(int)reader["SubgroupID"],

(string)reader["Title"],

(string)reader["Manager"],

(string)reader["Description"]);   <----- THIS LINE IS WHERE THE FAILUER OCCURS!

return list;