diff --git a/lib/plug/conn.ex b/lib/plug/conn.ex
index ecb79c0b..90961d48 100644
--- a/lib/plug/conn.ex
+++ b/lib/plug/conn.ex
@@ -1437,7 +1437,7 @@ defmodule Plug.Conn do
 
   defp adapter_inform(%Conn{adapter: {adapter, payload}}, status, headers) do
     for {key, value} <- headers do
-      validate_header_key_value!(key, value)
+      validate_header_key_value!(to_string(key), value)
     end
 
     adapter.inform(payload, status, headers)
diff --git a/test/plug/conn_test.exs b/test/plug/conn_test.exs
index fba5caae..d5080944 100644
--- a/test/plug/conn_test.exs
+++ b/test/plug/conn_test.exs
@@ -468,6 +468,11 @@ defmodule Plug.ConnTest do
     end
   end
 
+  test "inform/3 accepts atom header keys (e.g. for websocket upgrades)" do
+    conn = conn(:get, "/foo") |> inform(101, [{:upgrade, "websocket"}, {:connection, "Upgrade"}])
+    assert {101, [{:upgrade, "websocket"}, {:connection, "Upgrade"}]} in sent_informs(conn)
+  end
+
   test "inform!/3 performs an informational request" do
     conn = conn(:get, "/foo") |> inform!(103, [{"link", "</style.css>; rel=preload; as=style"}])
     assert {103, [{"link", "</style.css>; rel=preload; as=style"}]} in sent_informs(conn)