diff --git a/lib/plug_require_header.ex b/lib/plug_require_header.ex
index dabb924..4647545 100644
--- a/lib/plug_require_header.ex
+++ b/lib/plug_require_header.ex
@@ -97,14 +97,14 @@ defmodule PlugRequireHeader do
         conn
       else
         conn
-        |> put_resp_header("content-type", content_type_for(format))
+        |> put_resp_content_type(content_type_for format)
         |> send_resp(status, format_message(message, format))
         |> halt
       end
     end
   end
 
-  defp content_type_for(:text), do: "text/plain; charset=utf-8"
+  defp content_type_for(:text), do: "text/plain"
   defp content_type_for(:json), do: "application/json"
 
   defp format_message(message, :text), do: message
diff --git a/test/plug_require_header_test.exs b/test/plug_require_header_test.exs
index 4eaf769..dfffc03 100644
--- a/test/plug_require_header_test.exs
+++ b/test/plug_require_header_test.exs
@@ -99,7 +99,7 @@ defmodule PlugRequireHeaderTest do
 
     assert response.status == 418
     assert response.resp_body == Poison.encode! %{error: "I'm a teapot!"}
-    assert content_type(response) == "application/json"
+    assert content_type(response) == "application/json; charset=utf-8"
   end
 
   defp content_type(response) do