diff --git a/core/lib/Drupal/Core/Config/Schema/ArrayElement.php b/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
index 1ae3ff0..19423dc 100644
--- a/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
+++ b/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Config\Schema;
 
+use Drupal\Component\Utility\String;
 use Drupal\Core\Config\TypedConfigManagerInterface;
 use Drupal\Core\TypedData\TraversableTypedDataInterface;
 use Drupal\Core\TypedData\TypedData;
@@ -94,7 +95,7 @@ public function get($name) {
       return $element;
     }
     else {
-      throw new \InvalidArgumentException(String::format("The configuration property @key doesn't exist.", array('@key' => $property_name)));
+      throw new \InvalidArgumentException(String::format("The configuration property @key doesn't exist.", array('@key' => $name)));
     }
   }
 
diff --git a/core/lib/Drupal/Core/Config/Schema/Mapping.php b/core/lib/Drupal/Core/Config/Schema/Mapping.php
index 5adfc20..7a055bf 100644
--- a/core/lib/Drupal/Core/Config/Schema/Mapping.php
+++ b/core/lib/Drupal/Core/Config/Schema/Mapping.php
@@ -7,9 +7,6 @@
 
 namespace Drupal\Core\Config\Schema;
 
-use Drupal\Core\TypedData\ComplexDataInterface;
-use Drupal\Component\Utility\String;
-
 /**
  * Defines a mapping configuration element.
  *