diff --git a/src/Plugin/Mail/SwiftMailer.php b/src/Plugin/Mail/SwiftMailer.php
index 3e89311..1d5d0a3 100644
--- a/src/Plugin/Mail/SwiftMailer.php
+++ b/src/Plugin/Mail/SwiftMailer.php
@@ -627,7 +627,7 @@ class SwiftMailer implements MailInterface, ContainerFactoryPluginInterface {
     // and then treat the result as safe markup.
     $line_endings = Settings::get('mail_line_endings', PHP_EOL);
     $applicable_format = $this->getApplicableFormat($message);
-    $filter_format = $this->config['message']['filter_format'];
+    $filter_format = isset($this->config['message']['filter_format']) ? $this->config['message']['filter_format'] : filter_fallback_format();
     $message['body'] = Markup::create(implode($line_endings, array_map(function ($body) use ($applicable_format, $filter_format) {
       // If the body contains no html tags but the applicable format is HTML,
       // we can assume newlines will need be converted to <br>.
diff --git a/swiftmailer.install b/swiftmailer.install
index 2e48116..fb889b9 100644
--- a/swiftmailer.install
+++ b/swiftmailer.install
@@ -27,6 +27,7 @@ function swiftmailer_requirements($phase) {
  */
 function swiftmailer_install() {
   $settings = Drupal::configFactory()->getEditable('mailsystem.settings');
+  $settings->set('filter_format', 'plain_text');
   $settings->set('modules.swiftmailer.none', [
     'formatter' => 'swiftmailer',
     'sender' => 'swiftmailer',