patch to chuned.rb

diff --git a/lib/rack/chunked.rb b/lib/rack/chunked.rb
index dddf969..0ee8fb9 100644
--- a/lib/rack/chunked.rb
+++ b/lib/rack/chunked.rb
@@ -35,7 +35,7 @@ module Rack
     def each
       term = "\r\n"
       @body.each do |chunk|
-        size = bytesize(chunk)
+        size = chunk.nil? ? 0 : bytesize(chunk)
         next if size == 0
         yield [size.to_s(16), term, chunk, term].join
       end