diff --git a/composer/schema/grid/interfaces.py b/composer/schema/grid/interfaces.py
index b9cb4af..2c9b9fb 100644
--- a/composer/schema/grid/interfaces.py
+++ b/composer/schema/grid/interfaces.py
@@ -40,6 +40,7 @@ class Grid(schema.List):
                               instanceName='grid'))
 
     column_types = []
+    cardinality = None
 
 
 class Records(Grid):