Be sure to initialize @heartbeat_timer. Third time is a charm!

lib/action_cable/server/connections.rb

@@ -22,8 +22,8 @@ def remove_connection(connection)
       # then can't rely on being able to receive and send to it. So there's a 3 second heartbeat running on all connections. If the beat fails, we automatically
       # disconnect.
       def setup_heartbeat_timer
-        @heartbeat_timer ||= EM.next_tick do
-          EventMachine.add_periodic_timer(BEAT_INTERVAL) do
+        EM.next_tick do
+          @heartbeat_timer ||= EventMachine.add_periodic_timer(BEAT_INTERVAL) do
             EM.next_tick { connections.map &:beat }
           end
         end