diff --git a/ntcore/src/main/native/include/networktables/ProtobufTopic.h b/ntcore/src/main/native/include/networktables/ProtobufTopic.h
index 362d2cb5b5b..5fe615b26d6 100644
--- a/ntcore/src/main/native/include/networktables/ProtobufTopic.h
+++ b/ntcore/src/main/native/include/networktables/ProtobufTopic.h
@@ -172,8 +172,8 @@ class ProtobufSubscriber : public Subscriber {
   }
 
  private:
-  wpi::mutex m_mutex;
-  wpi::ProtobufMessage<T> m_msg;
+  mutable wpi::mutex m_mutex;
+  mutable wpi::ProtobufMessage<T> m_msg;
   ValueType m_defaultValue;
 };