diff --git a/pubsub/envelope.go b/pubsub/envelope.go
index 4a4d048af..d57a8793c 100644
--- a/pubsub/envelope.go
+++ b/pubsub/envelope.go
@@ -103,6 +103,19 @@ func FromCloudEvent(cloudEvent []byte, topic, pubsub, traceID string) (map[strin
 	m[TopicField] = topic
 	m[PubsubField] = pubsub
 
+	// specify default value if it's unspecified from the original CloudEvent
+	if m[SourceField] == nil {
+		m[SourceField] = DefaultCloudEventSource
+	}
+
+	if m[TypeField] == nil {
+		m[TypeField] = DefaultCloudEventType
+	}
+
+	if m[SpecVersionField] == nil {
+		m[SpecVersionField] = CloudEventsSpecVersion
+	}
+
 	return m, nil
 }