diff --git a/core/modules/config_translation/src/FormElement/ListElement.php b/core/modules/config_translation/src/FormElement/ListElement.php
index d7a082f..9d65d2f 100644
--- a/core/modules/config_translation/src/FormElement/ListElement.php
+++ b/core/modules/config_translation/src/FormElement/ListElement.php
@@ -135,20 +135,19 @@ protected function getGroupTitle(DataDefinitionInterface $definition, array $gro
   }
 
   /**
-   * Get compound element key.
+   * Gets a compound element key.
    *
-   * @param $base_key
-   *   The base key.
-   *
-   * @param $key
+   * @param string|null $base_key
+   *   The base key or NULL of this is the top-level element.
+   * @param string $key
    *   The sub-key.
    *
    * @return string
+   *   The compound element key.
    */
   protected function getElementKey($base_key, $key) {
-    return implode('.', array_filter(array($base_key, $key), function ($value) {
-      return isset($value);
-    }));
+    // Account for the fact that $base_key might be NULL.
+    return ltrim("$base_key.$key", '.');
   }
 
 }