diff --git a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
index 724d875..1a8a08a 100644
--- a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
@@ -90,8 +90,14 @@ public function testMimeHeader($value, $encoded) {
   public function providerTestMimeHeader() {
     return array(
       array('tést.txt', '=?UTF-8?B?dMOpc3QudHh0?='),
+      // String longer than 47 characters including special characters. The
+      // result is an encoded chunk splitted in two parts separated by a CLRF
+      // and a space.
+      array('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaà', "=?UTF-8?B?YWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFh?=\r\n  =?UTF-8?B?YWHDoA==?="),
       // Simple ASCII characters.
       array('ASCII', 'ASCII'),
+      // Long ASCII string (more than 47 characters).
+      array('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa', 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'),
     );
   }