diff --git a/test/transcoding/bookstore-server.cc b/test/transcoding/bookstore-server.cc
index 5abc026a1..216004d18 100644
--- a/test/transcoding/bookstore-server.cc
+++ b/test/transcoding/bookstore-server.cc
@@ -27,6 +27,7 @@
 #include <algorithm>
 #include <iostream>
 #include <map>
+#include <mutex>
 #include <string>
 #include <vector>
 
@@ -47,6 +48,8 @@ void PrintRequest(const MessageType& message) {
   static ::google::protobuf::util::TypeResolver* type_resolver =
       ::google::protobuf::util::NewTypeResolverForDescriptorPool(
           "type.googleapis.com", message.GetDescriptor()->file()->pool());
+  static std::mutex mutex;
+  std::lock_guard<std::mutex> lock(mutex);
 
   std::string binary;
   if (!message.SerializeToString(&binary)) {