Test assert_broadcast_on without specifying the message content

I am writing tests for a Rails 6 app.

I have an action, post "/action", that implies messages broadcasted to a channel, user_channel1.

I tried to use assert_broadcast_on in my integration tests.

assert_broadcast_on("user_channel1") do
      post "/action"
end
# Error: ArgumentError: wrong number of arguments (given 1, expected 2) ...

I would like to know if it's possible to test that messages are broadcasted without giving the exact messages content. Thanks!

Note: messages can be json formatted.