diff --git a/core/modules/language/src/Form/NegotiationUrlForm.php b/core/modules/language/src/Form/NegotiationUrlForm.php
index 49110a3..2bf56d5 100644
--- a/core/modules/language/src/Form/NegotiationUrlForm.php
+++ b/core/modules/language/src/Form/NegotiationUrlForm.php
@@ -144,7 +144,7 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
     // Count repeated values for uniqueness check.
     $count = array_count_values($form_state->getValue('prefix'));
     $default_langcode = $this->config('language.negotiation')->get('selected_langcode');
-    if ($default_langcode == LanguageInterface::LANGCODE_SITE_DEFAULT) {
+    if ($default_langcode == LanguageInterface::LANGCODE_SITE_DEFAULT || !isset($default_langcode)) {
       $default_langcode = $this->languageManager->getDefaultLanguage()->getId();
     }
     foreach ($languages as $langcode => $language) {