diff --git a/Gemfile b/Gemfile
index c86cabb..e7e94e5 100644
--- a/Gemfile
+++ b/Gemfile
@@ -18,7 +18,7 @@ group :test do
 end
 
 group :doc do
-  gem "redcarpet"
+  gem "redcarpet", :platform => :mri
   gem "yard"
 end
 
diff --git a/lib/http/form_data/composite_io.rb b/lib/http/form_data/composite_io.rb
index 61c72ad..30d556d 100644
--- a/lib/http/form_data/composite_io.rb
+++ b/lib/http/form_data/composite_io.rb
@@ -9,7 +9,7 @@ class CompositeIO
       # @param [Array<IO>] ios Array of IO objects
       def initialize(ios)
         @index  = 0
-        @buffer = String.new
+        @buffer = "".b
         @ios    = ios.map do |io|
           if io.is_a?(String)
             StringIO.new(io)
@@ -29,14 +29,16 @@ def initialize(ios)
       #
       # @return [String, nil]
       def read(length = nil, outbuf = nil)
-        outbuf = outbuf.to_s.replace("")
+        outbuf = outbuf.to_s.clear
+        # buffer in JRuby is sometimes US-ASCII, force to ASCII-8BIT
+        outbuf.force_encoding(Encoding::BINARY)
 
         while current_io
           current_io.read(length, @buffer)
-          outbuf << @buffer
+          outbuf << @buffer.force_encoding(Encoding::BINARY)
 
           if length
-            length -= @buffer.length
+            length -= @buffer.bytesize
             break if length.zero?
           end
 
diff --git a/lib/http/form_data/multipart/param.rb b/lib/http/form_data/multipart/param.rb
index 080b9e2..5972752 100644
--- a/lib/http/form_data/multipart/param.rb
+++ b/lib/http/form_data/multipart/param.rb
@@ -62,7 +62,7 @@ def self.coerce(data)
         private
 
         def header
-          header = String.new
+          header = "".b
           header << "Content-Disposition: form-data; #{parameters}#{CRLF}"
           header << "Content-Type: #{content_type}#{CRLF}" if content_type
           header << CRLF