The problem occurs when one of the options Options.WORKER_WRITE_THREADS or Options.WORKER_READ_THREADS equals 0.
In that case, the worker has only one type of thread (either only read threads, or only write threads), and the user is stuck with choosing that type of thread for establishing connections. In other words, if Options.WORKER_WRITE_THREADS is 0, setting WORKER_ESTABLISH_WRITING to false (which is the default value) is mandatory. Similarly, if Options.WORKER_READ_THREADS is 0, Options.WORKER_ESTABLISH_WRITING must be true.