diff -u b/core/modules/migrate/src/MigrateExecutable.php b/core/modules/migrate/src/MigrateExecutable.php
--- b/core/modules/migrate/src/MigrateExecutable.php
+++ b/core/modules/migrate/src/MigrateExecutable.php
@@ -604,7 +604,7 @@
     else {
       $type = 'error';
     }
-    $source_id_string = implode($event->getSourceIdValues());
+    $source_id_string = implode(',', $event->getSourceIdValues());
     $this->message->display($source_id_string . ': ' . $event->getMessage(), $type);
   }