diff --git a/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php b/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php
index e9a7664..122e9ba 100644
--- a/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php
+++ b/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php
@@ -28,11 +28,12 @@ public static function getLabel() {
    */
   public function getContext($query_arg = NULL) {
     if ($query_arg === NULL) {
-      return $this->requestStack->getCurrentRequest()->getQueryString();
-    }
-    else {
-      return $this->requestStack->getCurrentRequest()->query->get($query_arg);
+      $query_string = $this->requestStack->getCurrentRequest()->getQueryString();
+      return $query_string === NULL ? '' : $query_string;
     }
+
+    $value = $this->requestStack->getCurrentRequest()->query->get($query_arg);
+    return $value === NULL ? '' : http_build_query([$query_arg => $value], '', '&');
   }
 
 }