diff --git a/core/lib/Drupal/Core/Plugin/CategorizingPluginManagerTrait.php b/core/lib/Drupal/Core/Plugin/CategorizingPluginManagerTrait.php
index 5a608e6..6cb8c5a 100644
--- a/core/lib/Drupal/Core/Plugin/CategorizingPluginManagerTrait.php
+++ b/core/lib/Drupal/Core/Plugin/CategorizingPluginManagerTrait.php
@@ -106,7 +106,7 @@ public function getGroupedDefinitions(array $definitions = NULL, $label_key = 'l
     $definitions = $this->getSortedDefinitions(isset($definitions) ? $definitions : $this->getDefinitions(), $label_key);
     $grouped_definitions = array();
     foreach ($definitions as $id => $definition) {
-      $grouped_definitions[$definition['category']][$id] = $definition;
+      $grouped_definitions[(string) $definition['category']][$id] = $definition;
     }
     return $grouped_definitions;
   }