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.