"session_start()" warning when using perch_template() with content containing as...

I'm getting the following error when using perch_template() to process some text-based content containing a [cms:asset 9999] shortcode. The problem does not occur if the content does not contain a shortcode.

Warning: session_start(): Cannot send session cache limiter - headers already sent in C:\xampp\htdocs\miraviva\perch\core\lib\PerchSession.class.php on line 27

Here's the line that creates the problem:

$country_body = perch_template('content/textile.html', array( 'text' => $country['country_body'] ), true);

The template it's referring to is a very simple one:

<perch:content id="text" type="textarea" label="Text" textile="true" />

Any ideas?

Diagnostics:

    Perch: 3.0.11, PHP: 5.6.30, MySQL: mysqlnd 5.0.11-dev - 20120503 - $Id: 76b08b24596e12d4553bd41fc93cccd5bac2fe7a $, with PDO
    Server OS: WINNT, apache2handler
    Installed apps: content (3.0.11), assets (3.0.11), categories (3.0.11), perch_blog (5.6.1), perch_forms (1.10), perch_gallery (2.8.9)
    App runtimes: <?php $apps_list = [ 'perch_blog', 'perch_gallery', 'perch_forms' ];
    PERCH_LOGINPATH: /perch
    PERCH_PATH: C:\xampp\htdocs\miraviva\perch
    PERCH_CORE: C:\xampp\htdocs\miraviva\perch\core
    PERCH_RESFILEPATH: C:\xampp\htdocs\miraviva\perch\resources
    Image manipulation: GD
    PHP limits: Max upload 128M, Max POST 128M, Memory: 128M, Total max file upload: 128M
    F1: 3b606135b33e6a102526838f4152a807
    Resource folder writeable: Yes
    HTTP_HOST: miraviva
    DOCUMENT_ROOT: C:/xampp/htdocs/miraviva
    REQUEST_URI: /perch/core/settings/diagnostics/
    SCRIPT_NAME: /perch/core/settings/diagnostics/index.php