diff -u b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
--- b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
@@ -286,9 +286,14 @@
    * Implements the magic __wakeup() method.
    */
   public function __wakeup() {
-    // Run the values from getExportProperties() through __construct().
-    $values = array_intersect_key($this->getExportProperties(), get_object_vars($this));
-    $this->__construct($values, $this->entityTypeId);
+    // PHPUnit uses unserilalize() on a made up string as a trick when bypassing
+    // the contructor on mock object creation. Make sure we only run on objects
+    // that have been serialized through our __sleep() method.
+    if (isset($this->entityTypeId)) {
+      // Run the values from getExportProperties() through __construct().
+      $values = array_intersect_key($this->getExportProperties(), get_object_vars($this));
+      $this->__construct($values, $this->entityTypeId);
+    }
   }
 
 }