Amazon: How get price for "Kindle" items?

I use http://ift.tt/1lBjeDz for updating all prices of my amazon.DE items. That working fine, but not for "Kindle" items.

I use for "Kindle" items:

    $lookup = new Lookup();
    $lookup->setItemId(implode(',',$items));
    $lookup->setCategory('KindleStore');
    $lookup->setResponseGroup(array('Offers'));

Result is:

[Items] => Array
    (
        [Request] => Array
            (
                [IsValid] => True
                [ItemLookupRequest] => Array
                    (
                        [IdType] => ASIN
                        [ItemId] => B00R4LYY7S
                        [ResponseGroup] => Offers
                        [VariationPage] => All
                    )

            )

        [Item] => Array
            (
                [ASIN] => B00R4LYY7S
            )

    )

Without $lookup->setCategory('KindleStore') and a normal ASIN for a Blu-ray the result look like:

[Items] => Array
    (
        [Request] => Array
            (
                [IsValid] => True
                [ItemLookupRequest] => Array
                    (
                        [IdType] => ASIN
                        [ItemId] => B00QT9M9O6
                        [ResponseGroup] => Offers
                        [VariationPage] => All
                    )

            )

        [Item] => Array
            (
                [ASIN] => B00QT9M9O6
                [OfferSummary] => Array
                    (
                        [LowestNewPrice] => Array
                            (
                                [Amount] => 1799
                                [CurrencyCode] => EUR
                                [FormattedPrice] => EUR 17,99
                            )

                        [TotalNew] => 1
                        [TotalUsed] => 0
                        [TotalCollectible] => 0
                        [TotalRefurbished] => 0
                    )

                [Offers] => Array
                    (
                        [TotalOffers] => 1
                        [TotalOfferPages] => 1
                        [MoreOffersUrl] => ...
                        [Offer] => Array
                            (
                                [OfferAttributes] => Array
                                    (
                                        [Condition] => New
                                    )

                                [OfferListing] => Array
                                    (
                                        [OfferListingId] => ...
                                        [Price] => Array
                                            (
                                                [Amount] => 1799
                                                [CurrencyCode] => EUR
                                                [FormattedPrice] => EUR 17,99
                                            )

                                        [Availability] => Noch nicht veröffentlicht
                                        [AvailabilityAttributes] => Array
                                            (
                                                [AvailabilityType] => now
                                                [IsPreorder] => 1
                                                [MinimumHours] => 48
                                                [MaximumHours] => 72
                                            )

                                        [IsEligibleForSuperSaverShipping] => 1
                                    )

                            )

                    )

            )

    )

)

Is this a Bug in Amazon? Can anybody help me here?

http://ift.tt/1BdyXVw http://ift.tt/1xSsGfE