uninitialized value in concatenation Plack::App::File

I got
"Use of uninitialized value in concatenation (.) or string at
/usr/share/perl5/Plack/App/File.pm line 30."
in nginx unit log every time the static content is requested.


https://github.com/plack/Plack/issues/472

If I understood correctly, @miyagawa says the Lint middleware should be
used, but (as I replied there) wrapping sub with
Plack::Middleware::Lint->wrap doesn't help.


Does anyone has same issue?


-- 
sergio.