diff --git a/core/modules/migrate/src/Plugin/migrate/ConfigurablePluginTrait.php b/core/modules/migrate/src/Plugin/migrate/ConfigurablePluginTrait.php
index af63cc7c82..b792030189 100644
--- a/core/modules/migrate/src/Plugin/migrate/ConfigurablePluginTrait.php
+++ b/core/modules/migrate/src/Plugin/migrate/ConfigurablePluginTrait.php
@@ -32,6 +32,8 @@ public function getConfiguration() {
    * @see \Drupal\Component\Plugin\ConfigurablePluginInterface::setConfiguration()
    */
   public function setConfiguration(array $configuration) {
+    // For migrate configurations it is important to preserve integer keys
+    // so we use mergeDeepArray here.
     $this->configuration = NestedArray::mergeDeepArray([$this->defaultConfiguration(), $configuration], TRUE);
   }