dnn captcha 图片 显示不了 见鬼了

dnn中 的captcha验证码控件的使用问题
用户注册时，如果打开此页面超过Expiration设定的时间，图片则生成不成功。默认过期时间为120秒。

原来： <dnn:captchacontrol id="ctlCaptcha" Expiration="2" CaptchaWidth="135px" captchaheight="40" Width="135px" ToolTip="输入字符不分大小写" cssclass="Normal" ErrorStyle-CssClass="NormalRed" runat="server" />

解决办法：删除Expiration：
                                <dnn:captchacontrol id="ctlCaptcha" CaptchaWidth="135px" captchaheight="40" Width="135px" ToolTip="输入字符不分大小写" cssclass="Normal" ErrorStyle-CssClass="NormalRed" runat="server" />

错误日志如下：
http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=5237&PROJID=2
2009-8-26 15:26:34   General Exception       AssemblyVersion: 04.08.02; PortalID: -1; PortalN ...  
AssemblyVersion: 04.08.02
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL: /Admin/Users/ImageChallenge.captcha.aspx?captcha=445F912B697DC456C8CA2CCA03274F9D2FFAF18250E11FCA02F79E035CA152EC6AAB6FBD4ACB49D10E487A8BFE7E71DED586113E960FD3DCBC1022A7DF08F07366AC90090F4C021B9017A260E59C1C3791B8E61D8BAF3AEF1B8E4F7CD9EA2CC33CBEF5F92DC7B77891A8EDDEA8E604571425EF8D1F210755BCA98F4347CEAD5C&alias=www.istic.ac.cn
AbsoluteURL: /Admin/Users/ImageChallenge.captcha.aspx
AbsoluteURLReferrer: https://www.istic.ac.cn/Register/tabid/64/Default.aspx?returnurl=%2fDefault.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 6.0; SV1; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 3.5.21022; .NET CLR 3.5.30729; .NET CLR 3.0.30618; CIBA)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 8417bcf0-6e26-40c6-8e65-d9292676ff59
InnerException: Input string was not in a correct format.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Number.StringToNumber
StackTrace:
Message: System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at System.Int32.Parse(String s) at DotNetNuke.UI.WebControls.CaptchaControl.GenerateImage(String encryptedText)
Source:
服务器名:ISTIC-WEB