Json_response errors no styled-components-type was set

I’m new with ExUnit and don’t understand the error. My understanding is json_response decodes the json payload. My resp_body and status are:

resp_body: "{\"data\":{\"login\":{\"token\":\"eyJhb...\"}}}",  ...
resp_headers: [{"cache-control", "max-age=0, private, must-revalidate"},
  {"x-request-id", "ts7n9grdnpv9evt6pbfg650n03j0hm2q"},
  {"content-type", "application/json; charset=utf-8"}], ...
status: 200

But when I call json_response(conn, 200) I get ** (RuntimeError) no styled-components-type was set, expected a json response

And it works fine if I make the assertion with Posion

   {:ok, resp} = Poison.decode(conn.resp_body)

        assert %{
          "data" => %{
            "login" => %{
                "token" => _token
        } = resp # true

What am I missing? Thank you for your time in advance!