diff --git a/lib/excon/middlewares/hijack.rb b/lib/excon/middlewares/hijack.rb
index cc7f71bd1..727cf2928 100644
--- a/lib/excon/middlewares/hijack.rb
+++ b/lib/excon/middlewares/hijack.rb
@@ -1,4 +1,6 @@
 module Excon
+  VALID_REQUEST_KEYS << :hijack_block
+
   module Middleware
     # Hijack is an Excon middleware which parses response headers and then
     # yields the underlying TCP socket for raw TCP communication (used to