diff -u b/core/misc/ajax.js b/core/misc/ajax.js
--- b/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -142,7 +142,7 @@
     // We don't need readyState except for status == 0.
     readyStateText = xmlhttp.status === 0 ? ('\n' + Drupal.t('ReadyState: !readyState', {'!readyState': xmlhttp.readyState})) : '';
 
-    customMessage = customMessage ? ("\n" + Drupal.t("CustomMessage: !customMessage", {'!customMessage': customMessage})) : "";
+    customMessage = customMessage ? ('\n' + Drupal.t('CustomMessage: !customMessage', {'!customMessage': customMessage})) : '';
 
     /**
      * Formatted and translated error message.
@@ -406,7 +406,7 @@
         //   the response headers cannot be accessed for verification.
         if (response !== null && !drupalSettings.ajaxTrustedUrl[ajax.url]) {
           if (xmlhttprequest.getResponseHeader('X-Drupal-Ajax-Token') !== '1') {
-            var customMessage = Drupal.t("The response failed verification so will not be processed.");
+            var customMessage = Drupal.t('The response failed verification so will not be processed.');
             return ajax.error(xmlhttprequest, ajax.url, customMessage);
           }
         }