diff --git a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
index 7e4c0bc..aca7611 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -239,6 +239,11 @@ public function useCaches($use_caches = FALSE) {
    * method.
    */
   public function processDefinition(&$definition, $plugin_id) {
+    // Only arrays can be operated on.
+    if (!is_array($definition)) {
+      return;
+    }
+
     if (!empty($this->defaults) && is_array($this->defaults)) {
       $definition = NestedArray::mergeDeep($this->defaults, $definition);
     }