Could not check origin for `Phoenix.Socket` in `runtime.exs` with `System.get_env/1`

Hi everyone!

For months now, we have used check_origin for production in runtime.exs like :

  app_name =
    System.get_env("FLY_APP_NAME") ||
      raise "FLY_APP_NAME not available"

  domain =
    System.get_env("DOMAIN") ||
      raise "DOMAIN not available"

config :phoenix_app, PhoenixAppWeb.Endpoint,
    server: true,
    url: [host: domain, port: 80],
    check_origin: [

But now we have the error Could not check origin for Phoenix.Socket transport in production’s logs.

Complete error log
 [info] 09:45:19.943 [error] Could not check origin for Phoenix.Socket transport.
 [info] Origin of the request: https://[Our domain]

 [info] This happens when you are attempting a socket connection to

 [info] a different host than the one configured in your config/

 [info] files. For example, in development the host is configured

 [info] to "localhost" but you may be trying to access it from

 [info] "". To fix this issue, you may either:

 [info] 1. update [url: [host: ...]] to your actual host in the

 [info] config file for your current environment (recommended)

 [info] 2. pass the :check_origin option when configuring your

 [info] endpoint or when configuring the transport in your

 [info] UserSocket module, explicitly outlining which origins

 [info] are allowed:

 [info] check_origin: ["",

 [info] "//", "//"]

Do you know what could have changed and how I can fix it?

Thanks for your help !

:recon_trace.calls({Phoenix.Socket.Transport, :check_origin, :return_trace}, 1000) could bring more light.