diff --git a/lib/rack/lint.rb b/lib/rack/lint.rb
index 0c6e1a35d..41aa6ea72 100644
--- a/lib/rack/lint.rb
+++ b/lib/rack/lint.rb
@@ -432,7 +432,7 @@ class HijackWrapper
 
       REQUIRED_METHODS = [
         :read, :write, :read_nonblock, :write_nonblock, :flush, :close,
-        :close_read, :close_write, :closed?
+        :close_read, :close_write, :closed?, :to_io
       ]
 
       def_delegators :@io, *REQUIRED_METHODS