UNMANAGED ERROR IN APPLICATION

Sorry, but the system encountered an unmanaged error.
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<ExceptionInformation><Exception ExceptionType=''System.InvalidOperationException'' Message=''Nullable object must have a value.'' Data=''System.Collections.ListDictionaryInternal'' TargetSite=''Void ThrowInvalidOperationException(System.ExceptionResource)'' Source=''mscorlib''><StackTrace> at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource) at System.Nullable`1.get_Value() at Website.Controls.MultiCatItem.Page_Load(Object sender, EventArgs e) at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)</StackTrace></Exception></ExceptionInformation>