diff --git a/settings.php b/settings.php
index b4954aab0..e14700bbb 100644
--- a/settings.php
+++ b/settings.php
@@ -129,8 +129,10 @@
 
 if (defined('PANTHEON_ENVIRONMENT')) {
   if (in_array($_ENV['PANTHEON_ENVIRONMENT'], array('dev', 'test', 'live'))) {
-    $settings['trusted_host_patterns'][] = "{$_ENV['PANTHEON_ENVIRONMENT']}-{$_ENV['PANTHEON_SITE_NAME']}.pantheon.io";
-    $settings['trusted_host_patterns'][] = "{$_ENV['PANTHEON_ENVIRONMENT']}-{$_ENV['PANTHEON_SITE_NAME']}.pantheonsite.io";
+    if (isset($settings['trusted_host_patterns'])) {
+      $settings['trusted_host_patterns'][] = "{$_ENV['PANTHEON_ENVIRONMENT']}-{$_ENV['PANTHEON_SITE_NAME']}.pantheon.io";
+      $settings['trusted_host_patterns'][] = "{$_ENV['PANTHEON_ENVIRONMENT']}-{$_ENV['PANTHEON_SITE_NAME']}.pantheonsite.io";
+    }
   }
 }