diff -u b/core/lib/Drupal/Component/Utility/UrlHelper.php b/core/lib/Drupal/Component/Utility/UrlHelper.php
--- b/core/lib/Drupal/Component/Utility/UrlHelper.php
+++ b/core/lib/Drupal/Component/Utility/UrlHelper.php
@@ -435,6 +435,10 @@
   public static function parseString($string) {
     $result = parse_query($string);
 
+    $result = array_map(function ($value) {
+      return $value === null ? '' : $value;
+    }, $result);
+
     $phpArrayParts = [];
     foreach ($result as $key => $value) {
       if (preg_match('/\[.*\]$/', $key)) {
diff -u b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
--- b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
@@ -623,7 +623,7 @@
    */
   public function testParseString($queryString, $queryArray) {
     $result = UrlHelper::parseString($queryString);
-    $this->assertEquals($queryArray, $result);
+    $this->assertSame($queryArray, $result);
   }
 
   /**
@@ -635,6 +635,7 @@
    */
   public function providerTestParseString() {
     $data = [
+      ['one', ['one' => '']],
       ['one=1', ['one' => '1']],
       ['one=1&two=2', ['one' => '1', 'two' => '2']],
       ['o%20e=1&two=2', ['o e' => '1', 'two' => '2']],