diff --git a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
index 91bbcb4..13bd3b6 100644
--- a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
@@ -25,7 +25,9 @@
  * - batch_size: (optional) Number of records to fetch from the database during
  *   each batch. If omitted, all records are fetched in a single query.
  * - ignore_map: (optional) Source data is joined to the map table by default.
- *   If set to TRUE, the map table will not be joined.
+ *   If set to TRUE, the map table will not be joined. If the SQL query uses
+ *   expressions, the ignore_map option must be set to TRUE because expressions
+ *   with aliases cannot be used with JOINs.
  *
  * For other optional configuration keys inherited from the parent class, refer
  * to \Drupal\migrate\Plugin\migrate\source\SourcePluginBase.