diff --git a/quiche/quic/tools/quic_spdy_client_base.h b/quiche/quic/tools/quic_spdy_client_base.h
index aad229a..ba62c80 100644
--- a/quiche/quic/tools/quic_spdy_client_base.h
+++ b/quiche/quic/tools/quic_spdy_client_base.h
@@ -96,7 +96,7 @@
   void SendRequestsAndWaitForResponse(const std::vector<std::string>& url_list);
 
   // Returns a newly created QuicSpdyClientStream.
-  QuicSpdyClientStream* CreateClientStream();
+  virtual QuicSpdyClientStream* CreateClientStream();
 
   // Returns a the session used for this client downcasted to a
   // QuicSpdyClientSession.