Elixir GenServer doesn't work with module attributes?

I am setting up a GenServer that runs every hour to cache data. I followed the example José Valim posted is response to a question about how to do this.

How to run some code every few hours in Phoenix framework?

It works great except that if the interval I gave the call to Process.send_after/3 is a module attribute it does not run. Does anyone have insights as to why this might be?

So the following does not work but if in the calls to Process.send_after/3 I replace @interval with 60 * 60 * 1000 it runs as expected (this is essentially the code from the other SO post above):

defmodule MyApp.Periodically do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, %{})
  end

  @interval 60 * 60 * 1000

  def init(state) do
    Process.send_after(self(), :work, @interval)
    {:ok, state}
  end

  def handle_info(:work, state) do
    # Begin caching the new data.
    MyApp.CacheManager.cache_new_data()

    # Start the timer again
    Process.send_after(self(), :work, @interval)
    {:noreply, state}
  end
end