diff --git a/data/avro-schema/src/main/resources/avro/net/corda/data/flow/event/session/SessionData.avsc b/data/avro-schema/src/main/resources/avro/net/corda/data/flow/event/session/SessionData.avsc
index 93993d54df..d92838430d 100644
--- a/data/avro-schema/src/main/resources/avro/net/corda/data/flow/event/session/SessionData.avsc
+++ b/data/avro-schema/src/main/resources/avro/net/corda/data/flow/event/session/SessionData.avsc
@@ -13,7 +13,8 @@
     },
     {
       "name": "sessionInit",
-      "type": "net.corda.data.flow.event.session.SessionInit"
+      "type": ["null", "net.corda.data.flow.event.session.SessionInit"],
+      "doc": "Contains information that can be used to start an initiated flow, piggybacked on initial data messages. Will be null for messages sent to the initiator. Will be null when initiated party is confirmed to be present via receipt of any data message previously."
     }
   ]
 }