Cannot open /usr/ports/Mk/bsd.port.mk

root@momh167-gjp4-hpelitebook8570p-freebsd:~ # date ; uname -v
Mon 29 Oct 2018 21:49:46 GMT
FreeBSD 13.0-CURRENT r339737 GENERIC-NODEBUG
root@momh167-gjp4-hpelitebook8570p-freebsd:~ # ls -hl /usr/ports
total 2304
-rw-r--r--   1 root  wheel    85B 29 Sep  2016 .arcconfig
-rw-r--r--   1 root  wheel   996B 18 Apr  2018 .gitattributes
-rw-r--r--   1 root  wheel    35K 22 Apr  2018 .gitauthors
-rw-r--r--   1 root  wheel   103B  4 Jan  2018 .gitignore
-rw-r--r--   1 root  wheel   1.4K  1 Jan  2018 .gitmessage
-rw-r--r--   1 root  wheel   2.4M 30 Apr  2018 .portsnap.INDEX
drwxr-xr-x  36 root  wheel   1.0K 29 Oct 21:40 distfiles
root@momh167-gjp4-hpelitebook8570p-freebsd:~ # svn co svn://svn.freebsd.org/ports/head/www /usr/ports/www
A    /usr/ports/www/p5-Dancer2-Plugin-Deferred
…
A    /usr/ports/www/g-cows/distinfo
Checked out revision 483402.
root@momh167-gjp4-hpelitebook8570p-freebsd:~ # svn cp svn://svn.freebsd.org/ports/head/www/waterfox@480899 /usr/ports/www/waterfox
A    /usr/ports/www/waterfox/Makefile
A    /usr/ports/www/waterfox/pkg-message
A    /usr/ports/www/waterfox/distinfo
A    /usr/ports/www/waterfox/files
A    /usr/ports/www/waterfox/files/patch-bug1431285
…
Checked out revision 480899.
A         /usr/ports/www/waterfox
root@momh167-gjp4-hpelitebook8570p-freebsd:~ # cd /usr/ports/www/waterfox
root@momh167-gjp4-hpelitebook8570p-freebsd:/usr/ports/www/waterfox # make configure
make: "/usr/share/mk/bsd.port.mk" line 32: Cannot open /usr/ports/Mk/bsd.port.mk
make: Fatal errors encountered -- cannot continue
make: stopped in /usr/ports/www/waterfox
root@momh167-gjp4-hpelitebook8570p-freebsd:/usr/ports/www/waterfox # cd ../firefox
root@momh167-gjp4-hpelitebook8570p-freebsd:/usr/ports/www/firefox # make configure
make: "/usr/share/mk/bsd.port.mk" line 32: Cannot open /usr/ports/Mk/bsd.port.mk
make: Fatal errors encountered -- cannot continue
make: stopped in /usr/ports/www/firefox
root@momh167-gjp4-hpelitebook8570p-freebsd:/usr/ports/www/firefox #