Weird error with phoenix_live_view on Heroku

I’m getting a super weird error when deploying an app with phoenix_live_view on Heroku:

09:33:03.143 [error] Ranch listener ReporterWeb.Endpoint.HTTP had connection
process started with :cowboy_clear:start_link/4 at #PID<0.576.0> exit with
reason: {:function_clause, [{Phoenix.Socket.Message, :from_map!, [["8", "8",
"lv:phx-oZOgz4XU", "phx_join", nil]], []}, {Phoenix.Socket, :__in__, 2, [file:
'lib/phoenix/socket.ex', line: 564]}, {Phoenix.Endpoint.Cowboy2Handler,
:websocket_handle, 2, [file: 'lib/phoenix/endpoint/cowboy2_handler.ex', line:
93]}, {:cowboy_websocket, :handler_call, 6, [file:
'/tmp/build_cae110e886c5c6dcb00c2dec21bb2a2f/deps/cowboy/src/cowboy_websocket.erl',
line: 482]}, {:cowboy_http, :loop, 1, [file:
'/tmp/build_cae110e886c5c6dcb00c2dec21bb2a2f/deps/cowboy/src/cowboy_http.erl',
line: 231]}, {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line:
249]}]}

Any idea what could be wrong here?