diff --git a/src/stream.c b/src/stream.c
index 40f9f75ae..0afa3059f 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -3102,6 +3102,9 @@ struct ist stream_generate_unique_id(struct stream *strm, struct lf_expr *format
 		if ((unique_id = pool_alloc(pool_head_uniqueid)) == NULL)
 			return IST_NULL;
 
+		/* Initialize ->unique_id to an empty string to prevent infinite
+		 * recursion when the <format> references %[unique-id] or %ID.
+		 */
 		strm->unique_id = ist2(unique_id, 0);
 		strm->unique_id.len = build_logline(strm, unique_id, UNIQUEID_LEN, format);