diff --git a/lib/net/http/generic_request.rb b/lib/net/http/generic_request.rb
index 96396b0a..5071d2ad 100644
--- a/lib/net/http/generic_request.rb
+++ b/lib/net/http/generic_request.rb
@@ -341,7 +341,7 @@ def encode_multipart_form_data(out, params, opt)
     end
     chunked_p = chunked?
 
-    buf = +''
+    buf = ''.b
     params.each do |key, value, h={}|
       key = quote_string(key, charset)
       filename =
@@ -349,7 +349,7 @@ def encode_multipart_form_data(out, params, opt)
         value.respond_to?(:to_path) ? File.basename(value.to_path) :
         nil
 
-      buf << "--#{boundary}\r\n"
+      buf << "--#{boundary}\r\n".b
       if filename
         filename = quote_string(filename, charset)
         type = (h[:content_type] || 'application/octet-stream').to_s
@@ -358,10 +358,10 @@ def encode_multipart_form_data(out, params, opt)
         end
         buf << "Content-Disposition: form-data; " \
           "name=\"#{key}\"; filename=\"#{filename}\"\r\n" \
-          "Content-Type: #{type}\r\n\r\n"
+          "Content-Type: #{type}\r\n\r\n".b
         if !out.respond_to?(:write) || !value.respond_to?(:read)
           # if +out+ is not an IO or +value+ is not an IO
-          buf << (value.respond_to?(:read) ? value.read : value)
+          buf << (value.respond_to?(:read) ? value.read : value).b
         elsif value.respond_to?(:size) && chunked_p
           # if +out+ is an IO and +value+ is a File, use IO.copy_stream
           flush_buffer(out, buf, chunked_p)
@@ -378,12 +378,12 @@ def encode_multipart_form_data(out, params, opt)
         #   HTML5 says, "The parts of the generated multipart/form-data
         #   resource that correspond to non-file fields must not have a
         #   Content-Type header specified."
-        buf << "Content-Disposition: form-data; name=\"#{key}\"\r\n\r\n"
-        buf << (value.respond_to?(:read) ? value.read : value)
+        buf << "Content-Disposition: form-data; name=\"#{key}\"\r\n\r\n".b
+        buf << (value.respond_to?(:read) ? value.read : value).b
       end
       buf << "\r\n"
     end
-    buf << "--#{boundary}--\r\n"
+    buf << "--#{boundary}--\r\n".b
     flush_buffer(out, buf, chunked_p)
     out << "0\r\n\r\n" if chunked_p
   end
diff --git a/test/net/http/test_http.rb b/test/net/http/test_http.rb
index 590f8a2f..140a614b 100644
--- a/test/net/http/test_http.rb
+++ b/test/net/http/test_http.rb
@@ -897,6 +897,46 @@ def test_set_form
     }
   end
 
+  def test_set_form_multipart_with_mixed_encodings
+    utf8 = "r\u{e4}ksm\u{f6}rg\u{e5}s".freeze
+    binary = "r\u{e4}kmacka".b.freeze
+    data = [
+      ['utf8', StringIO.new(utf8)],
+      ["bin\u{e4}ry", StringIO.new(binary)],
+      ['file', utf8, filename: "f\u{e4}le"],
+      ['file', binary, filename: 'binary']
+    ]
+    expected = <<"__EOM__".b.gsub(/\n/, "\r\n")
+--<boundary>
+Content-Disposition: form-data; name="utf8"
+
+r\u{e4}ksm\u{f6}rg\u{e5}s
+--<boundary>
+Content-Disposition: form-data; name="bin\u{e4}ry"
+
+r\u{e4}kmacka
+--<boundary>
+Content-Disposition: form-data; name="file"; filename="f\u{e4}le"
+Content-Type: application/octet-stream
+
+r\u{e4}ksm\u{f6}rg\u{e5}s
+--<boundary>
+Content-Disposition: form-data; name="file"; filename="binary"
+Content-Type: application/octet-stream
+
+r\u{e4}kmacka
+--<boundary>--
+__EOM__
+    [false, true].each do |chunked|
+      start {|http|
+        @server.mount('/', lambda {|req, res| res.body = req.body })
+        _test_set_form_multipart(http, chunked, data, expected)
+      }
+    end
+    assert_equal(Encoding::UTF_8, utf8.encoding)
+    assert_equal(Encoding::BINARY, binary.encoding)
+  end
+
   def _test_set_form_urlencoded(http, data)
     req = Net::HTTP::Post.new('/')
     req.set_form(data)