diff --git a/dockerbuild/ssp-overrides/config.php b/dockerbuild/ssp-overrides/config.php
index 7f0280c6..3e96a82f 100644
--- a/dockerbuild/ssp-overrides/config.php
+++ b/dockerbuild/ssp-overrides/config.php
@@ -271,7 +271,7 @@
      * Example:
      *   'trusted.url.domains' => ['sp.example.com', 'app.example.com'],
      */
-    'trusted.url.domains' => null,
+    'trusted.url.domains' => [],
 
     /*
      * Enable regular expression matching of trusted.url.domains.