ISN extension bug?

Hello devs,

ISTM that there is an issue on the ISMN type:
 sh> psql psql (9.3.2) Type "help" for help. # CREATE EXTENTION isn; # SELECT ISMN 'M123456782';  M-1234-5678-5

*** The 2 is changed to 5 in the display...
 # SELECT ISMN 'M123456785'; ERROR:  invalid check digit for ISMN number: "M123456785", should be 2 LINE 1: SELECT ISMN
'M123456785';                   ^ # SELECT ISMN 'M-1234-5678-5'; ERROR:  invalid check digit for ISMN number:
"M-1234-5678-5",should be 2 LINE 1: SELECT ISMN 'M-1234-5678-5';                    ^
 

-- 
Fabien.