ASP.NET C#字串转换问题

我写的程式如下：
SqlDataReader reader1; //宣告一个DataReader
SqlConnection connStr1 = new SqlConnection(WebConfigurationManager.ConnectionStrings["glory01ConnectionStringERP"].ConnectionString);
String select1 = "select Count(*) AS acount,convert(nvarchar(10),Sum((CASE WHEN 完工码 ='Y' THEN 1 Else 0 End))) as Y,convert(nvarchar(10),Sum((CASE WHEN 完工码 ='N' THEN 1 Else 0 End))) as N";
select1 += " from [glory01].[dbo].[SFCTA-VIEW] where (ID ='" + aID + "')";
try
{
connStr1.Open();
SqlCommand cmd1 = new SqlCommand(select1, connStr1);
reader1 = cmd1.ExecuteReader();
while (reader1.Read())
{
string a = reader1[0].ToString();
string b = reader1[1].ToString();
string c = reader1[2].ToString();
Response.Write("a=" + a + " ," + "b=" + b + " ," + "c=" + c + "");
int j = Convert.ToInt32(a);//转换字串改数字笔数
int k = Convert.ToInt32(b);//完工
int h = Convert.ToInt32(c);//未完工
Response.Write("j=" + j + " ," + "k=" + k + " ," + "h=" + h + "");
}

但执行后：「输入字串格式不正确。」指向：int k = Convert.ToInt32(b);//完工不了解为何无法转换为int请大大能帮我解题，谢谢。