diff --git a/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php b/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php
index 8ffeecd..a0f817f 100644
--- a/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php
+++ b/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php
@@ -31,11 +31,11 @@ public static function getLabel() {
   public function getContext($query_arg = NULL) {
     if ($query_arg === NULL) {
       $query_string = $this->requestStack->getCurrentRequest()->getQueryString();
-      return $query_string === NULL ? '' : $query_string;
+      return (string) $query_string;
     }
 
     $value = $this->requestStack->getCurrentRequest()->query->get($query_arg, NULL, TRUE);
-    return $value === NULL ? '' : http_build_query([$query_arg => $value], '', '&');
+    return (string) http_build_query([$query_arg => $value], '', '&');
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Cache/Context/QueryArgsCacheContextTest.php b/core/tests/Drupal/Tests/Core/Cache/Context/QueryArgsCacheContextTest.php
index f223d36..8e76480 100644
--- a/core/tests/Drupal/Tests/Core/Cache/Context/QueryArgsCacheContextTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/Context/QueryArgsCacheContextTest.php
@@ -33,7 +33,7 @@ public function testGetContext($uri, array $context_tokens, array $expected_keys
 
     $cache_contexts_manager = new CacheContextsManager($container_prophecy->reveal(), ['url.query_args']);
 
-    $this->assertSame($expected_keys, $cache_contexts_manager->convertTokensToKeys($context_tokens));
+    $this->assertSame($expected_keys, $cache_contexts_manager->convertTokensToKeys($context_tokens)->getKeys());
   }
 
   /**