Empty cover even if cover exists

[Flapane]

Hello,
let's say I'm reading this book: https://www.goodreads.com/book/show/1...

I want to parse all the books I'm reading (they are in the "currently-reading" shelf).
The prototype of the API xml call is: http://www.goodreads.com/review/list/...

The xml output tell us that there's no cover ( http://www.goodreads.com/assets/nocov... ), even if that version of the book has indeed a cover.
Please see the whole output at http://pastebin.com/ZaMJyLph

This is a strange behaviour that rarely happens with reading and read books, no matter the name of the shelf.
For example, the same bug happens with this book which is in my "read" shelf: https://www.goodreads.com/book/show/2...

[Ettore]

The problem is that depending on how we obtained the metadata for that particular book we may not be allowed to display the book cover via the api. Unfortunately this is a contractual/legal restriction and there's not much we can do about this.

[Flapane]

That makes sense.
Thanks for your answer.