diff --git a/quic/core/quic_session.h b/quic/core/quic_session.h
index d221fe6..fba1e63 100644
--- a/quic/core/quic_session.h
+++ b/quic/core/quic_session.h
@@ -51,7 +51,7 @@
   // An interface from the session to the entity owning the session.
   // This lets the session notify its owner (the Dispatcher) when the connection
   // is closed, blocked, or added/removed from the time-wait list.
-  class Visitor {
+  class QUIC_EXPORT_PRIVATE Visitor {
    public:
     virtual ~Visitor() {}