system.linq solves the exception Class cannot be indexed because it has no default property

here is my code

     Dim ContentSent As HashSet(Of String) = New HashSet(Of String)(SubscriberContent.ContentIdSent.Split(","))

    content = New Content
   '////////////here is the line that is causing this exception
    content = contentCollection.Find(Function(x) x.CntId = Convert.ToInt32(ContentSent(0)))

The weird thing is that when I import System.Linq the compilation error disappears.

any idea why ?