Error with websocket dispatch config

I am trying upgrade to Elixir 1.8, Erlang 21, and Phoenix 1.4. I’m getting an error with our custom http dispatch config, which was added when we were at Phoenix 1.3/Elixir 1.5.

Here is our config:

  config :site, SiteWeb.Endpoint,
    http: [
      dispatch: [
        {:_,
         [
           {"/socket/websocket", Phoenix.Endpoint.CowboyWebSocket,
            {Phoenix.Transports.WebSocket, {SiteWeb.Endpoint, SiteWeb.UserSocket, :websocket}}},
           {"/socket/longpoll", Plug.Cowboy.Handler,
            {Phoenix.Transports.LongPoll, {SiteWeb.Endpoint, SiteWeb.UserSocket, :longpoll}}},
           {:_, Plug.Cowboy.Handler, {SiteWeb.Endpoint, []}}
         ]}
      ]
    ]

And the error we’re seeing:

** (UndefinedFunctionError) function Phoenix.Endpoint.CowboyWebSocket.init/2 is undefined or private
    (phoenix) Phoenix.Endpoint.CowboyWebSocket.init(%{bindings: %{}, body_length: 0, cert: :undefined, has_body: false, headers: %{"accept-encoding" => "gzip, deflate, br", "accept-language" => "en-US,en;q=0.9", "cache-control" => "no-cache", "connection" => "upgrade", "cookie" => "_ga=GA1.2.323690242.1505750053; __insp_uid=4076678333; _ga=GA1.3.1382101712.1505318427; fs_uid=fullstory.com`AQKVD`5182813288529920:5629499534213120; __insp_wid=101978311; __insp_nv=false; __insp_targlpu=aHR0cHM6Ly9kZXYubWJ0YWNlLmNvbS8%3D; __insp_targlpt=TUJUQSAtIE1hc3NhY2h1c2V0dHMgQmF5IFRyYW5zcG9ydGF0aW9uIEF1dGhvcml0eQ%3D%3D; __insp_norec_sess=true; __insp_slim=1525973426869; mbta_id=32820746; _ga=GA1.4.323690242.1505750053; _site_key=SFMyNTY.g3QAAAAA.xxxBgGh7d8xGoy6cezO2La-1xvwRbt_RjIPutJlQj8U; _gid=GA1.3.930802726.1551714745; _gid=GA1.2.930802726.1551714745; mbta_visited_routes=CR-Fitchburg|85; _gid=GA1.4.930802726.1551714745; _gat_UA-82280837-1=1", "host" => "green.dev.mbtace.com", "origin" => "https://green.dev.mbtace.com", "pragma" => "no-cache", "sec-websocket-extensions" => "permessage-deflate; client_max_window_bits", "sec-websocket-key" => "IdIkX8mVrmEKVbPIdL0ZZw==", "sec-websocket-version" => "13", "upgrade" => "websocket", "user-agent" => "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.119 Safari/537.36", "x-amzn-trace-id" => "Root=1-5c7eece1-4c47339fb70d03ccbdd1d686", "x-forwarded-for" => "67.208.190.2, 67.208.190.2", "x-forwarded-port" => "443", "x-forwarded-proto" => "https", "x-real-ip" => "67.208.190.2"}, host: "green.dev.mbtace.com", host_info: :undefined, method: "GET", path: "/socket/websocket", path_info: :undefined, peer: {{172, 17, 0, 1}, 54138}, pid: #PID<0.6326.0>, port: 80, qs: "vsn=2.0.0", ref: SiteWeb.Endpoint.HTTP, scheme: "http", sock: {{172, 17, 0, 2}, 4000}, streamid: 9, version: :"HTTP/1.1"}, {Phoenix.Transports.WebSocket, {SiteWeb.Endpoint, SiteWeb.UserSocket, :websocket}})
    (cowboy) /root/deps/cowboy/src/cowboy_handler.erl:41: :cowboy_handler.execute/2
    (cowboy) /root/deps/cowboy/src/cowboy_stream_h.erl:296: :cowboy_stream_h.execute/3
    (cowboy) /root/deps/cowboy/src/cowboy_stream_h.erl:274: :cowboy_stream_h.request_process/3
    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
Initial Call: :cowboy_stream_h.request_process/3
Ancestors: [#PID<0.6326.0>, #PID<0.2391.0>, #PID<0.2390.0>, SiteWeb.Endpoint, Site.Supervisor, #PID<0.2377.0>]

As far as I can tell, Phoenix.Endpoint.CowboyWebSocket hasn’t changed in a while, so I’m guessing that there was some change to how the args are being passed to that init function? I’m struggling to follow the pipeline to even figure out how this is getting called. If anyone has any suggestions they would be greatly appreciated!