关于c＃：输入字符串的格式不正确，格式为1和2

Input string was not in a correct format , with form 1 and 2

为什么说mscorlib.dll中发生"system.formatException"类型的未处理异常

其他信息：输入字符串的格式不正确。

这是我的密码

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public int transCode = 0;

con.Open();
cmd = new SqlCommand("SELECT MAX(TransactionCode) FROM TRANSACTIONS",con);
rdr = cmd.ExecuteReader();

while (rdr.Read())
{
transCode = Convert.ToInt32(rdr[0].ToString()); // Here is the line where the problem go
}

con.Close();