Exception: Specified cast is not valid

Question

Can any1 tell me why the below line throws an exception? if ((DateTime.Now.TimeOfDay.CompareTo(((DateTime)RowSchedule["StartEnter"]).TimeOfDay) >= 0) && (DateTime.Now.TimeOfDay.CompareTo(((DateTime)RowTurno["LimiteEnter"]).TimeOfDay) <= 0)) The RowSchedule is a DataRow that retrieve the data from a Table from a DataSet. The table have 6 DateTime columns. This is the excepton: System.InvalidCastException Additional information: Specified cast is not valid. Can any1 tell me what is wrong? Thax in advance.