diff --git a/upb/conformance/BUILD b/upb/conformance/BUILD
index 9630c016bc..e645cc202b 100644
--- a/upb/conformance/BUILD
+++ b/upb/conformance/BUILD
@@ -73,6 +73,7 @@ cc_binary(
         "//editions:test_messages_proto3_editions_upbdefs",
         "//upb:base",
         "//upb:json",
+        "//upb:message",
         "//upb:port",
         "//upb:reflection",
         "//upb:text",
@@ -127,6 +128,7 @@ cc_binary(
         "//editions:test_messages_proto3_editions_upbdefs",
         "//upb:base",
         "//upb:json",
+        "//upb:message",
         "//upb:port",
         "//upb:reflection",
         "//upb:text",
diff --git a/upb/conformance/conformance_upb.c b/upb/conformance/conformance_upb.c
index 349d9ec83f..28c713e8c5 100644
--- a/upb/conformance/conformance_upb.c
+++ b/upb/conformance/conformance_upb.c
@@ -23,6 +23,7 @@
 #include "upb/base/upcast.h"
 #include "upb/json/decode.h"
 #include "upb/json/encode.h"
+#include "upb/message/message.h"
 #include "upb/reflection/message.h"
 #include "upb/text/encode.h"
 #include "upb/wire/decode.h"
@@ -68,6 +69,7 @@ typedef struct {
 } ctx;
 
 bool parse_proto(upb_Message* msg, const upb_MessageDef* m, const ctx* c) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
   upb_StringView proto =
       conformance_ConformanceRequest_protobuf_payload(c->request);
   if (upb_Decode(proto.data, proto.size, msg, upb_MessageDef_MiniTable(m), NULL,
@@ -117,6 +119,7 @@ void serialize_text(const upb_Message* msg, const upb_MessageDef* m,
 }
 
 bool parse_json(upb_Message* msg, const upb_MessageDef* m, const ctx* c) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
   upb_StringView json = conformance_ConformanceRequest_json_payload(c->request);
   upb_Status status;
   int opts = 0;
@@ -172,6 +175,7 @@ void serialize_json(const upb_Message* msg, const upb_MessageDef* m,
 }
 
 bool parse_input(upb_Message* msg, const upb_MessageDef* m, const ctx* c) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
   switch (conformance_ConformanceRequest_payload_case(c->request)) {
     case conformance_ConformanceRequest_payload_protobuf_payload:
       return parse_proto(msg, m, c);