diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index ff5e96fdf71c0e2c5a578e1a248895579e3adbb5..a5d082c6a20e10340890b0545188d83bc91cd265 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -43,6 +43,7 @@ def initialize(status = 200, header = {}, body = [])
         @writer = lambda { |x| @body << x }
         @block = nil
         @length = 0
+        @_etag = nil
 
         @status, @header = status, header
         self.body = body