Minserve log error: "Use of uninitialized value"


I see the below error being reported constantly in /var/webmin/miniserv.error:

Use of uninitialized value in string eq at /usr/libexec/webmin/virtual-server-theme/newleft.cgi line 173

It seems to be accessing the Virtualmin ui which triggers it so I post it here.

But everything seems to work well (as always!), and there are no obvious errors in the UI.

system is webmin 1.782 + vmin 5.0 gpl on centos 7.
Using the standard Virtualmin framed theme (Authentic is nice, but its very heavy lifting for my slow adsl)