Validator fails on this ISBN10 number

[tmaier]

1.9.3-p286 :001 > require "isbn"
 => true 
1.9.3-p286 :002 > ISBN.valid? "3-540-49698-x"
 => false 

But this book exists

https://portal.dnb.de/opac.htm?method=simpleSearch&query=3-540-49698-x