diff --git a/README.md b/README.md
index cb23f38..8cecf65 100644
--- a/README.md
+++ b/README.md
@@ -121,6 +121,7 @@ classDiagram
         timestamp : TimestampVectorData
         duration : DurationVectorData, optional
         meanings_tables : list[MeaningsTable]
+        [additional_columns] : list[VectorData], optional
     }
 
     class NdxEventsNWBFile {