diff -u b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
--- b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
@@ -254,7 +254,8 @@
         }
         else {
           try {
-            // Make the request. Guzzle throws an exception for anything but 200.
+            // Make the request.
+            // Guzzle throws an exception for anything but 200.
             \Drupal::httpClient()->head($source);
           }
           catch (TransferException $e) {