Update

lib/nulla_web/controllers/webfinger_controller.ex

@@ -7,7 +7,7 @@ defmodule NullaWeb.WebfingerController do
   def index(conn, %{"resource" => resource}) do
     case Regex.run(~r/^acct:(.+)@(.+)$/, resource) do
       [_, username, domain] ->
-        case Actor.get_actor(username, domain) do
+        case Actor.get_actor(preferredUsername: username, domain: domain) do
           nil ->
             conn
             |> put_resp_content_type("text/plain")