diff --git a/lib/plug_require_header.ex b/lib/plug_require_header.ex
index 637bb86..dabb924 100644
--- a/lib/plug_require_header.ex
+++ b/lib/plug_require_header.ex
@@ -97,7 +97,7 @@ defmodule PlugRequireHeader do
         conn
       else
         conn
-        |> put_resp_header("Content-Type", content_type_for(format))
+        |> put_resp_header("content-type", content_type_for(format))
         |> send_resp(status, format_message(message, format))
         |> halt
       end
diff --git a/test/plug_require_header_test.exs b/test/plug_require_header_test.exs
index c8bde55..4eaf769 100644
--- a/test/plug_require_header_test.exs
+++ b/test/plug_require_header_test.exs
@@ -103,6 +103,6 @@ defmodule PlugRequireHeaderTest do
   end
 
   defp content_type(response) do
-    hd get_resp_header(response, "Content-Type")
+    hd get_resp_header(response, "content-type")
   end
 end