[InvalidOperationException: Sequence contains no elements]
System.Linq.Enumerable.Single(IEnumerable`1 source) +320
System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__3(IEnumerable`1 sequence) +40
System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.ExecuteSingle(IEnumerable`1 query, Expression queryRoot) +60
System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute(Expression expression) +113
System.Data.Entity.Internal.Linq.DbQueryProvider.Execute(Expression expression) +105
System.Linq.Queryable.Single(IQueryable`1 source) +220
knowledges.Page_Load(Object sender, EventArgs e) +659
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +52
System.Web.UI.Control.OnLoad(EventArgs e) +97
System.Web.UI.Control.LoadRecursive() +61
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +693
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