diff --git a/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManager.php b/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManager.php
index ddd4799..9ff525b 100644
--- a/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManager.php
+++ b/core/lib/Drupal/Core/StreamWrapper/StreamWrapperManager.php
@@ -23,6 +23,10 @@ class StreamWrapperManager extends ContainerAware {
   /**
    * Contains stream wrapper info.
    *
+   * An associative array where keys are scheme names and values are themselves
+   * associative arrays with the keys class, type and (optionally) service_id,
+   * and string values.
+   *
    * @var array
    */
   protected $info = array();
@@ -30,16 +34,20 @@ class StreamWrapperManager extends ContainerAware {
   /**
    * Contains collected stream wrappers.
    *
-   * @var array
-   */
-  protected $wrappers = array();
-
-  /**
-   * Contains tagged stream-wrapper services.
+   * Keyed by filter, each value is itself an associative array keyed by scheme.
+   * Each of those values is an array representing a stream wrapper, with the
+   * following keys and values:
+   *   - class: stream wrapper class name
+   *   - type: a bitmask corresponding to the type constants in
+   *     StreamWrapperInterface
+   *   - service_id: name of service
+   *
+   * The array on key StreamWrapperInterface::ALL contains representations of
+   * all schemes and corresponding wrappers.
    *
    * @var array
    */
-  protected $services = array();
+  protected $wrappers = array();
 
   /**
    * The module handler.
@@ -231,8 +239,8 @@ public function getClass($scheme) {
    *   A stream wrapper object, or false if the scheme is not available.
    */
   protected function getWrapper($scheme, $uri) {
-    if (isset($this->services[$scheme])) {
-      $instance = $this->container->get($this->services[$scheme]);
+    if (isset($this->info[$scheme]['service_id'])) {
+      $instance = $this->container->get($this->info[$scheme]['service_id']);
       $instance->setUri($uri);
       return $instance;
     }
@@ -249,13 +257,15 @@ protected function getWrapper($scheme, $uri) {
    *   The service id.
    * @param string $class
    *   The stream wrapper class.
+   * @param string $scheme
+   *   The scheme for which the wrapper should be registered.
    */
   public function addStreamWrapper($service_id, $class, $scheme) {
-    $info = array();
-    $info['class'] = $class;
-    $info['type'] = $class::getType();
-    $this->services[$scheme] = $service_id;
-    $this->info[$scheme] = $info;
+    $this->info[$scheme] = array(
+      'class' => $class,
+      'type' => $class::getType(),
+      'service_id' => $service_id,
+    );
   }
 
   /**
@@ -265,7 +275,6 @@ public function addStreamWrapper($service_id, $class, $scheme) {
    */
   public function register() {
     $this->moduleHandler->alter('stream_wrappers', $this->info);
-    $this->services = array_intersect_key($this->services, $this->info);
 
     foreach ($this->info as $scheme => $info) {
       $this->registerWrapper($scheme, $info['class'], $info['type']);
@@ -278,8 +287,14 @@ public function register() {
    * Internal use only.
    */
   public function unregister() {
-    foreach (array_keys($this->wrappers[StreamWrapperInterface::ALL]) as $scheme) {
-      stream_wrapper_unregister($scheme);
+    // Normally, there are definitely wrappers set for the ALL filter. However,
+    // in some cases involving many container rebuilds (e.g. WebTestBase),
+    // $this->wrappers may be empty although wrappers are still registered
+    // globally. Thus an isset() check is needed before iterating.
+    if (isset($this->wrappers[StreamWrapperInterface::ALL])) {
+      foreach (array_keys($this->wrappers[StreamWrapperInterface::ALL]) as $scheme) {
+        stream_wrapper_unregister($scheme);
+      }
     }
   }