diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index a9f7ad0..cde113c 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -335,7 +335,7 @@ protected function drupalLogin(AccountInterface $account) {
     }
 
     $edit = array(
-      'name' => $account->getDisplayName(),
+      'name' => $account->getAccountName(),
       'pass' => $account->pass_raw
     );
     $this->drupalPostForm('user/login', $edit, t('Log in'));
@@ -344,7 +344,7 @@ protected function drupalLogin(AccountInterface $account) {
     if (isset($this->sessionId)) {
       $account->session_id = $this->sessionId;
     }
-    $pass = $this->assert($this->drupalUserIsLoggedIn($account), format_string('User %account successfully logged in.', array('%account' => $account->getAccountName())), 'User login');
+    $pass = $this->assert($this->drupalUserIsLoggedIn($account), new FormattableMarkup('User %name successfully logged in.', array('%name' => $account->getDisplayName())), 'User login');
     if ($pass) {
       $this->loggedInUser = $account;
       $this->container->get('current_user')->setAccount($account);
diff --git a/core/modules/user/src/Tests/UserLoginTest.php b/core/modules/user/src/Tests/UserLoginTest.php
index 2d66fc7..e79faff 100644
--- a/core/modules/user/src/Tests/UserLoginTest.php
+++ b/core/modules/user/src/Tests/UserLoginTest.php
@@ -156,7 +156,7 @@ function testPasswordRehashOnLogin() {
    */
   function assertFailedLogin($account, $flood_trigger = NULL) {
     $edit = array(
-      'name' => $account->getDisplayName(),
+      'name' => $account->getAccountName(),
       'pass' => $account->pass_raw,
     );
     $this->drupalPostForm('user/login', $edit, t('Log in'));
diff --git a/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
index eac054f..b7649128 100644
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -716,13 +716,13 @@ protected function drupalLogin(AccountInterface $account) {
     $this->drupalGet('user/login');
     $this->assertSession()->statusCodeEquals(200);
     $this->submitForm(array(
-      'name' => $account->getDisplayName(),
+      'name' => $account->getAccountName(),
       'pass' => $account->passRaw,
     ), t('Log in'));
 
     // @see BrowserTestBase::drupalUserIsLoggedIn()
     $account->sessionId = $this->getSession()->getCookie($this->getSessionName());
-    $this->assertTrue($this->drupalUserIsLoggedIn($account), new FormattableMarkup('User %name successfully logged in.', array('%name' => $account->getAccountName())));
+    $this->assertTrue($this->drupalUserIsLoggedIn($account), new FormattableMarkup('User %name successfully logged in.', array('%name' => $account->getDisplayName())));
 
     $this->loggedInUser = $account;
     $this->container->get('current_user')->setAccount($account);