diff --git a/embulk-output-redshift/README.md b/embulk-output-redshift/README.md
index 13011f66..c5d0d414 100644
--- a/embulk-output-redshift/README.md
+++ b/embulk-output-redshift/README.md
@@ -98,7 +98,7 @@ Redshift output plugin for Embulk loads records to Redshift.
   * Transactional: Yes.
   * Resumable: No.
 * **merge**:
-  * Behavior: This mode writes rows to some intermediate tables first. If all those tasks run correctly, inserts new records from intermediate tables after updating records whose keys exist in intermediate tables. Namely, if merge keys of a record in the intermediate tables already exist in the target table, the target record is updated by the intermediate record, otherwise the intermediate record is inserted. If the target table doesn't exist, it is created automatically.
+  * Behavior: This mode writes rows to some intermediate tables first. If all those tasks run correctly, inserts new records from intermediate tables after updating records whose keys exist in intermediate tables. Namely, if merge keys of a record in the intermediate tables already exist in the target table, the target record is updated by the intermediate record, otherwise the intermediate record is inserted. If the target table doesn't exist, it is created automatically. NOTE: Merge does not work correctly if merge keys contain `NULL`s.
   * Transactional: Yes.
   * Resumable: Yes.