diff --git a/validator/src/main/java/com/google/daq/mqtt/validator/Validator.java b/validator/src/main/java/com/google/daq/mqtt/validator/Validator.java
index 91e6f3b43d..3c62f12d72 100644
--- a/validator/src/main/java/com/google/daq/mqtt/validator/Validator.java
+++ b/validator/src/main/java/com/google/daq/mqtt/validator/Validator.java
@@ -565,7 +565,7 @@ private void sendValidationMessage(String deviceId, Object message, String topic
 
   private void writeMessageCapture(Map<String, Object> message, Map<String, String> attributes) {
     String deviceId = attributes.get("deviceId");
-    String type = attributes.get("subType");
+    String type = attributes.getOrDefault("subType", UNKNOWN_TYPE_DEFAULT);
     String folder = attributes.get("subFolder");
     AtomicInteger messageIndex = deviceMessageIndex.computeIfAbsent(deviceId,
         key -> new AtomicInteger());