diff --git a/lib/rack/session/cookie.rb b/lib/rack/session/cookie.rb
index 830a4e3..b724822 100644
--- a/lib/rack/session/cookie.rb
+++ b/lib/rack/session/cookie.rb
@@ -266,6 +266,10 @@ def write_session(req, session_id, session, options)
         session = session.merge("session_id" => session_id)
         session_data = encode_session_data(session)
 
+        if @legacy_hmac_secret
+          session_data << "--#{legacy_generate_hmac(session_data)}"
+        end
+
         if session_data.size > (4096 - @key.size)
           req.get_header(RACK_ERRORS).puts("Warning! Rack::Session::Cookie data size exceeds 4K.")
           nil