diff --git a/core/modules/system/src/Plugin/Block/SystemMainBlock.php b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
index beba38db4a8d4919430ba2d5f0a1714dde68bdc0..d54a1cae5a328ed36c41c883c41afc081cc11b7a 100644
--- a/core/modules/system/src/Plugin/Block/SystemMainBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
@@ -20,7 +20,7 @@ class SystemMainBlock extends BlockBase implements MainContentBlockPluginInterfa
    *
    * @var array
    */
-  protected $mainContent;
+  protected $mainContent = [];
 
   /**
    * {@inheritdoc}