diff --git a/core/lib/Drupal/Core/Session/AccountProxy.php b/core/lib/Drupal/Core/Session/AccountProxy.php
index 2c00e62..4fe3813 100644
--- a/core/lib/Drupal/Core/Session/AccountProxy.php
+++ b/core/lib/Drupal/Core/Session/AccountProxy.php
@@ -108,11 +108,11 @@ public function getAccount() {
    */
   public function impersonateAccount(AccountInterface $account) {
     // Prevent session information from being saved and push the previous account.
-    $this->accountStack[] = $this->account;
+    array_push($this->accountStack, $this->account);
     $this->originalSessionSaving = $this->sessionManager->isEnabled();
     $this->sessionManager->disable();
     $this->setAccount($account);
-    return $this->account;
+    return $this;
   }
 
   /**
@@ -123,30 +123,32 @@ public function revertAccount() {
     if (!empty($this->accountStack)) {
       $this->account = array_pop($this->accountStack);
     }
+    else {
+      throw new \RuntimeException(t('No more account impersonations to revert.'));
+    }
     // Restore original session saving status if all impersonations are reverted.
     if (empty($this->accountStack)) {
       if ($this->originalSessionSaving) {
         $this->sessionManager->enable();
       }
     }
-    return $this->account;
+    return $this;
   }
 
   /**
-  /**
    * {@inheritdoc}
    */
   public function revertAll() {
     // Restore the original account from the stack.
     if (!empty($this->accountStack)) {
-      $this->account = $this->accountStack[0];
+      $this->account = array_shift($this->accountStack);
       $this->accountStack = array();
     }
     // Restore original session saving status if all impersonations are reverted.
     if ($this->originalSessionSaving) {
       $this->sessionManager->enable();
     }
-    return $this->account;
+    return $this;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Session/AccountProxyInterface.php b/core/lib/Drupal/Core/Session/AccountProxyInterface.php
index b90ee99..b507150 100644
--- a/core/lib/Drupal/Core/Session/AccountProxyInterface.php
+++ b/core/lib/Drupal/Core/Session/AccountProxyInterface.php
@@ -41,7 +41,7 @@ public function getAccount();
    * call!
    *
    * @param \Drupal\Core\Session\AccountInterface
-   *   The account to be impersonated.
+   *   $this.
    */
   public function impersonateAccount(AccountInterface $account);
 
@@ -49,7 +49,7 @@ public function impersonateAccount(AccountInterface $account);
    * Revert from impersonating another account.
    *
    * @return \Drupal\Core\Session\AccountInterface
-   *   The last account that was impersonated.
+   *   $this.
    */
   public function revertAccount();
 
@@ -60,7 +60,7 @@ public function revertAccount();
    * been possibility of multiple impersonations.
    *
    * @return \Drupal\Core\Session\AccountInterface
-   *   The original account that was impersonated.
+   *   $this.
    */
   public function revertAll();
 }