diff --git a/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php b/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
index 89e602f..5a50152 100644
--- a/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
+++ b/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
@@ -145,6 +145,12 @@ public function render() {
    * @return int
    */
   protected function getPluralIndex() {
+    // If there is no translation in the target language, we need to use the
+    // default language's index.
+    if (!$this->getStringTranslation()->getStringTranslation($this->getOption('langcode'), $this->getUntranslatedString(), $this->getOption('context')) && $this->getOption('langcode')) {
+      return -1;
+    }
+
     // We have to test both if the function and the service exist since in
     // certain situations it is possible that locale code might be loaded but
     // the service does not exist. For example, where the parent test site has