diff --git a/spring-webflow/src/main/java/org/springframework/webflow/engine/impl/FlowSessionImpl.java b/spring-webflow/src/main/java/org/springframework/webflow/engine/impl/FlowSessionImpl.java
index 75659c501..c8f75eb18 100644
--- a/spring-webflow/src/main/java/org/springframework/webflow/engine/impl/FlowSessionImpl.java
+++ b/spring-webflow/src/main/java/org/springframework/webflow/engine/impl/FlowSessionImpl.java
@@ -157,7 +157,7 @@ public void readExternal(ObjectInput in) throws IOException, ClassNotFoundExcept
 	}
 
 	public void writeExternal(ObjectOutput out) throws IOException {
-		out.writeObject(flow.getId());
+		out.writeObject(flow != null ? flow.getId() : flowId);
 		out.writeObject(state != null ? state.getId() : null);
 		out.writeObject(scope);
 		out.writeObject(parent);