Why is Test::WWW::Mechanize::PSGI using a port?

I have some code that looks like this:

use SomeApp;
use Test::WWW::Mechanize::PSGI;                                                                                                                                         
my $mech = Test::WWW::Mechanize::PSGI->new(
    app  => sub { SomeApp->run(@_) },
);
$mech->get_ok('/');

However, as soon as get_ok() is called, I get the following warning:

PSGI error: failed to listen to port 8080: Address already in use at .../5.18.1/HTTP/Server/PSGI.pm line 94.
HTTP::Server::PSGI::setup_listener('HTTP::Server::PSGI=HASH(0x7fe6622fad60)') called at .../5.18.1/HTTP/Server/PSGI.pm line 54

And yes, I'm using that port for something else. From the docs of Test::WWW::Mechanize::PSGI:

This module allows you to test PSGI web applications but does not require a server or issue HTTP requests. Instead, it passes the HTTP request object directly to PSGI.

So in theory, I shouldn't need to specify a port, but I get the above warning and pages fetched return a 500 (they work fine in the browser). What am I missing?

• Test::WWW::Mechanize::PSGI version 0.35
• Plack version 1.0030
• Catalyst version 5.90051

Changing MyApp->run to MyApp->psgi_app results in:

Can't call method "request" on an undefined value at .../5.18.1/Test/WWW/Mechanize/PSGI.pm line 47.

This error can be replicated with:

catalyst.pl MyApp
cd MyApp
# run the test program above