Server Error in '/Portal' Application.

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Line 6:  {
Line 7:  	yaf_mhcip.PortalForum portalForum = new yaf_mhcip.PortalForum();
Line 8:  	Forum1.CategoryID = portalForum.GetPortalCategory("Forum");
Line 9:  
Line 10: 	

[FormatException: به تابع ToGuid مقدار غير معتبر ارسال شده است]
   Mafk.Portal.Classes.UIInput.ToGuid(String guidString) +209
   yaf.Config.GetPortalBoard(Nullable`1 categoryId) +66
   yaf_mhcip.PortalForum.GetPortalCategory(String type) +129
   ASP.forum_portalforum_showforum_aspx.Page_Load(Object sender, EventArgs e) in f:\QOM_WEB_350SP2\Public\Portal\Forum\PortalForum\showForum.aspx:8
   System.Web.UI.Control.OnLoad(EventArgs e) +106
   System.Web.UI.Control.LoadRecursive() +11905622
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3785