在线水平自测系统问题

做完题目，点击提交试卷，为什么会出现下边提示错误

Server Error in '/' Application.
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Input string was not in a correct format.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.FormatException: Input string was not in a correct format.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:


[FormatException: Input string was not in a correct format.]
System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) +7477367
System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) +119
Caicai.Jingpin.UI.Plugins.Exam.Answer_Click(Object sender, EventArgs e) +383
System.Web.UI.WebControls.Button.OnClick(EventArgs e) +78
System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +100
System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint

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