diff --git a/core/modules/simpletest/src/RandomGeneratorTrait.php b/core/modules/simpletest/src/RandomGeneratorTrait.php
index 2a22de2..5a50439 100644
--- a/core/modules/simpletest/src/RandomGeneratorTrait.php
+++ b/core/modules/simpletest/src/RandomGeneratorTrait.php
@@ -22,22 +22,25 @@
   protected $randomGenerator;
 
   /**
-   * Generates a unique random string of ASCII characters of codes 32 to 126.
+   * Generates a pseudo-random string of ASCII characters of codes 32 to 126.
    *
    * Do not use this method when special characters are not possible (e.g., in
    * machine or file names that have already been validated); instead, use
-   * randomMachineName().
+   * \Drupal\simpletest\TestBase::randomMachineName(). If $length is greater
+   * than 3 the random string will include at least one ampersand ('&') and
+   * at least one greater than ('>') character to ensure coverage for special
+   * characters and avoid the introduction of random test failures.
    *
    * @param int $length
    *   Length of random string to generate.
    *
    * @return string
-   *   Randomly generated unique string.
+   *   Pseudo-randomly generated unique string including special characters.
    *
    * @see \Drupal\Component\Utility\Random::string()
    */
   public function randomString($length = 8) {
-    if ($length < 3) {
+    if ($length < 4) {
       return $this->getRandomGenerator()->string($length, TRUE, array($this, 'randomStringValidate'));
     }
 
@@ -45,9 +48,10 @@ public function randomString($length = 8) {
     // returned string contains a character that needs to be escaped in HTML by
     // injecting an ampersand into it.
     $replacement_pos = floor($length / 2);
-    // Remove 1 from the length to account for the ampersand character.
-    $string = $this->getRandomGenerator()->string($length - 1, TRUE, array($this, 'randomStringValidate'));
-    return substr_replace($string, '&', $replacement_pos, 0);
+    // Remove 2 from the length to account for the ampersand and greater than
+    // characters.
+    $string = $this->getRandomGenerator()->string($length - 2, TRUE, array($this, 'randomStringValidate'));
+    return substr_replace($string, '>&', $replacement_pos, 0);
   }
 
   /**
diff --git a/core/modules/simpletest/tests/src/Unit/TestBaseTest.php b/core/modules/simpletest/tests/src/Unit/TestBaseTest.php
index f4a005f..cea275c 100644
--- a/core/modules/simpletest/tests/src/Unit/TestBaseTest.php
+++ b/core/modules/simpletest/tests/src/Unit/TestBaseTest.php
@@ -117,7 +117,7 @@ public function testRandomString($length) {
     $this->assertEquals($length, strlen($string));
     // randomString() should always include an ampersand ('&')  and a
     // greater than ('>') if $length is greater than 3.
-    if ($length > 3) {
+    if ($length > 4) {
       $this->assertContains('&', $string);
       $this->assertContains('>', $string);
     }