     ///     // Prints "Herman Melville"
     ///
     ///     let books: Set<Book> = row["books"]
-    ///     print(books[0].title)
+    ///     print(books.first!.title)
     ///     // Prints "Moby-Dick"
     public subscript<Record: FetchableRecord & Hashable>(_ key: String) -> Set<Record> {
         guard let rows = prefetchedRows[key] else {

    ///     print(country.name)
    ///     // Prints "United States"
    ///
    /// A fatal error is raised if the scope is not available.
    ///
    /// See https://github.com/groue/GRDB.swift/blob/master/README.md#joined-queries-support
    /// for more information.

        guard let scopedRow = scopesTree[scope] else {
            // Programmer error
            fatalError("missing scope `\(scope)` (row: \(self))")
        }
        guard scopedRow.containsNonNullValue else {
            // Programmer error
            fatalError("scope `\(scope)` only contains null values (row: \(self))")
        }
        return Record(row: scopedRow)
    }
