Error Caught in Page_Error event

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Error in: https://www.lullidancewear.com/shop/calla/full-sole-satin-ballet-shoes-for-beginners-cal406-calla
Error Message: Value cannot be null. Parameter name: source
Stack Trace:
at MoreLinq.MoreEnumerable.DistinctBy[TSource,TKey](IEnumerable`1 source, Func`2 keySelector, IEqualityComparer`1 comparer) at DAL.Models.Products.SelectByCn(String cn) at LulliDancewear.ProductPage.Page_Load(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)