diff --git a/core/misc/ajax.es6.js b/core/misc/ajax.es6.js
index 72745cc971..3ff762ade1 100644
--- a/core/misc/ajax.es6.js
+++ b/core/misc/ajax.es6.js
@@ -814,14 +814,14 @@
   /**
    * An animated progress throbber and container element for AJAX operations.
    *
-   * @param {string|null} message
+   * @param {string} [message]
    *   (optional) The message shown on the UI.
    * @return {string}
    *   The HTML markup for the throbber.
    */
-  Drupal.theme.ajaxProgressThrobber = (message = null) => {
+  Drupal.theme.ajaxProgressThrobber = (message) => {
     // Build markup without adding extra white space since it affects rendering.
-    const messageMarkup = message !== null ? Drupal.theme('ajaxProgressMessage', message) : '';
+    const messageMarkup = typeof message === 'string' ? Drupal.theme('ajaxProgressMessage', message) : '';
     const throbber = '<div class="throbber">&nbsp;</div>';
 
     return `<div class="ajax-progress ajax-progress-throbber">${throbber}${messageMarkup}</div>`;
@@ -843,7 +843,7 @@
    * @return {string}
    *   The HTML markup for the throbber.
    */
-  Drupal.theme.ajaxProgressMessage = (message) => `<div class="message">${message}</div>`;
+  Drupal.theme.ajaxProgressMessage = message => `<div class="message">${message}</div>`;
 
   /**
    * Sets the progress bar progress indicator.
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 938e115044..a95e041bd7 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -368,10 +368,8 @@ function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr
     }
   };
 
-  Drupal.theme.ajaxProgressThrobber = function () {
-    var message = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : null;
-
-    var messageMarkup = message !== null ? Drupal.theme('ajaxProgressMessage', message) : '';
+  Drupal.theme.ajaxProgressThrobber = function (message) {
+    var messageMarkup = typeof message === 'string' ? Drupal.theme('ajaxProgressMessage', message) : '';
     var throbber = '<div class="throbber">&nbsp;</div>';
 
     return '<div class="ajax-progress ajax-progress-throbber">' + throbber + messageMarkup + '</div>';