asp.net奇怪的问题

string =大<22222222>

int recipientSta = recipient.IndexOf("<") + 1;
int recipientEnd = recipient.IndexOf(">") - 1;
recipModel.FaxNum = recipient.Substring(recipientSta, recipientEnd);

上面的不会报错

string =louis<22222222>

int recipientSta = recipient.IndexOf("<") + 1;
int recipientEnd = recipient.IndexOf(">") - 1;
recipModel.FaxNum = recipient.Substring(recipientSta, recipientEnd);

怎么就回报错？

这是asp.NET么？

在C#里，定义是：string sNet="大<222222222>";这样的--

是都错.