ISBN's with a prefix of 979 ( such as 9799577623170 ) cannot be converted to ISBN-10s ( as per http://www.isbn.org/about_ISBN_standard ). When Koha tries to convert a 13 with a prefix of 979 to a 10, it gets back nothing, on which it calls as_string(). This is not good.
*** This bug has been marked as a duplicate of bug 13167 ***