Perch Layout in Email Template

Hi,

When using perch:layout, e.g.

<perch:layout path="emails/header" /> in a template, e.g. members/emails/welcome.html, I seem to get an error when sending out the email using the "Send welcome email" in the Perch Members app.

Uncaught Error: Call to undefined function perch_layout() in /admin/core/lib/PerchTemplate.class.php:1574

It's happening when working locally and remotely. Running latest Perch Runway. Any ideas?

Perch Runway: 3.1.2, PHP: 7.1.19, MySQL: mysqlnd 5.0.12-dev - 20150407 - $Id: 38fea24f2847fa7519001be390c98ae0acafe387 $, with PDO
Server OS: Darwin, apache2handler
Installed apps: content (3.1.2), assets (3.1.2), categories (3.1.2), perch_blog (5.6.1), perch_forms (1.10), perch_members (1.6.4), perch_mailchimp (3.1)
App runtimes: <?php $apps_list = [ 'perch_mailchimp', 'perch_blog', 'perch_forms', 'perch_members' ];
PERCH_LOGINPATH: /admin
PERCH_PATH: /admin
PERCH_CORE: /admin/core
PERCH_RESFILEPATH: /admin/resources
Image manipulation: GD
PHP limits: Max upload 32M, Max POST 32M, Memory: 128M, Total max file upload: 32M
F1: 3b606135b33e6a102526838f4152a807
Resource folder writeable: Yes
HTTP_HOST: localhost
DOCUMENT_ROOT: localhost
REQUEST_URI: /admin/core/settings/diagnostics/
SCRIPT_NAME: /admin/core/settings/diagnostics/index.php