C# MS Access数据库日期字段插入空值

private void button1_Click(object sender, EventArgs e)    
       {     
           OleDbConnection conn = new OleDbConnection(DAL.DBConnectionString.ConnectString);     
           OleDbCommand com = new OleDbCommand();     
           com.Connection = conn;     
           com.CommandText = "insert into aa(a,b) values(@a,@b)";     
           com.Parameters.Add(new OleDbParameter("@a", "a1"));     
           com.Parameters.Add(new OleDbParameter("@b", DBNull.Value));     
           conn.Open();     
           com.ExecuteNonQuery();         
           conn.Close();            OleDbCommand com2 = new OleDbCommand();    
           com2.Connection = conn;     
           conn.Open();     
           com2.CommandText = "insert into aa(a,b) values(@a,@b)";     
           com2.Parameters.Add(new OleDbParameter("@a", "a2"));     
           com2.Parameters.Add(new OleDbParameter("@b", Convert.ToDateTime("1992-1-1")));     
           com2.ExecuteNonQuery();     
           conn.Close();            OleDbCommand com23 = new OleDbCommand();    
           com23.Connection = conn;     
           conn.Open();     
           com23.CommandText = "insert into aa(a,b) values(@a,@b)";     
           com23.Parameters.Add(new OleDbParameter("@a", "a3"));     
           com23.Parameters.Add(new OleDbParameter("@b", "1993-1-1"));     
           com23.ExecuteNonQuery();     
           conn.Close();            MessageBox.Show("OK");    
       }