diff --git a/core/lib/Drupal/Core/Field/FieldItemBase.php b/core/lib/Drupal/Core/Field/FieldItemBase.php
index 5cadca2..b9c90d9 100644
--- a/core/lib/Drupal/Core/Field/FieldItemBase.php
+++ b/core/lib/Drupal/Core/Field/FieldItemBase.php
@@ -191,7 +191,14 @@ public function __isset($name) {
    * {@inheritdoc}
    */
   public function __unset($name) {
-    $this->set($name, NULL);
+    // Explicitly unset the property in $this->values if a non-defined
+    // property is unset, such that its key is removed from $this->values.
+    if (!$this->definition->getPropertyDefinition($name)) {
+      unset($this->values[$name]);
+    }
+    else {
+      $this->set($name, NULL);
+    }
   }
 
   /**