diff --git a/lib/puma/server.rb b/lib/puma/server.rb
index c9786dd6..90a17101 100644
--- a/lib/puma/server.rb
+++ b/lib/puma/server.rb
@@ -263,7 +263,7 @@ module Puma
 
         if res_body.kind_of? String
           if chunked
-            client.write res_body.size.to_s
+            client.write res_body.size.to_s(16)
             client.write line_ending
             client.write res_body
             client.write line_ending
@@ -275,7 +275,7 @@ module Puma
         else
           res_body.each do |part|
             if chunked
-              client.write part.size.to_s
+              client.write part.size.to_s(16)
               client.write line_ending
               client.write part
               client.write line_ending
diff --git a/test/test_persistent.rb b/test/test_persistent.rb
index eb455f3d..3317ddd8 100644
--- a/test/test_persistent.rb
+++ b/test/test_persistent.rb
@@ -55,4 +55,14 @@ class TestPersistent < Test::Unit::TestCase
     assert_equal "HTTP/1.1 200 OK\r\nTransfer-Encoding: chunked\r\nX-Header: Works\r\n\r\n5\r\nHello\r\n7\r\nChunked\r\n0\r\n", lines(9)
   end
 
+  def test_hex
+    str = "This is longer and will be in hex"
+    @body << str
+
+    @client << @valid_request
+
+    assert_equal "HTTP/1.1 200 OK\r\nTransfer-Encoding: chunked\r\nX-Header: Works\r\n\r\n5\r\nHello\r\n#{str.size.to_s(16)}\r\n#{str}\r\n0\r\n", lines(9)
+
+  end
+
 end