Make clean trying to unmount /dev on RPI2 in /usr/ports

[bob prohaska]

Lateley attempts to run make in /usr/ports has been generating
errors along the lines of
umount: unmount of /dev failed: No such file or directory
umount: retrying using path instead of file system ID

The first time it appeared was while running make index,
but it happened again running make -j4 clean. 

It's hard to believe this is intentional behavior, am I
mistaken? Svnlite info reports

Path: .
Working Copy Root Path: /usr/ports
URL: svn://svn.freebsd.org/ports/head
Relative URL: ^/head
Repository Root: svn://svn.freebsd.org/ports
Repository UUID: 35697150-7ecd-e111-bb59-0022644237b5
Revision: 454376
Node Kind: directory
Schedule: normal
Last Changed Author: wen
Last Changed Rev: 454376
Last Changed Date: 2017-11-17 07:11:59 -0800 (Fri, 17 Nov 2017)

Thanks for reading and any guidance,

bob prohaska