Making php56-pdo_pgsql fails

I previously managed to build php56-pdo_pgsql to be able to run Drupal 8 with PostgreSQL 9.6 after

echo 'DEFAULT_VERSIONS+=pgsql=9.6' >> /etc/make.conf

I'm now trying to do the same in a FreeBSD 10.3 jail running under a FreeNAS 9.10
but get
"/usr/ports/Mk/", line 1162: UNAME_r (10.3-STABLE) and OSVERSION (903000) do not agree on major version number.

Is there anything I can do to get around this?