diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 7820fa9..c532214 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1749,12 +1749,19 @@ protected function drupalProcessAjaxResponse($content, array $ajax_response, arr
           $wrapperNode = NULL;
           // When a command doesn't specify a selector, use #ajax['wrapper_selector'].
           if (!isset($command['selector'])) {
-            $command['selector'] = $ajax_settings['wrapper_selector'];
+            $selector_type = substr($ajax_settings['wrapper_selector'], 0, 1);
+            $selector = substr($ajax_settings['wrapper_selector'], 1);
+            if ($selector_type === '#') {
+              $wrapperNode = $xpath->query('//*[@id="' . $selector . '"]')->item(0);
+            }
+            else {
+              $wrapperNode = $xpath->query('//*[@class="' . $selector . '"]')->item(0);
+            }
           }
           // @todo Ajax commands can target any jQuery selector, but these are
           //   hard to fully emulate with XPath. For now, just handle 'head'
           //   and 'body', since these are used by the Ajax renderer.
-          if (in_array($command['selector'], array('head', 'body'))) {
+          elseif (in_array($command['selector'], array('head', 'body'))) {
             $wrapperNode = $xpath->query('//' . $command['selector'])->item(0);
           }
           if ($wrapperNode) {