Coroutines with Retrofit crashing with no stack trace when calling `RequestBody.create`

Question

I am using Retrofit with coroutines (built in support). I have a Retrofit interface, retrofit implementation and the API implementation created by Retrofit, but it doesn't matter as it doesn't get called. It crashes before that, here: In XService.kt

suspend fun test() {
    val mediaType = MediaType.parse("text/plain")
    val requestBody = RequestBody.create(mediaType, "TEST STRING")  // Crashes here
    // Now make some network requests
}

which is called by a viewModel:

    suspend fun test() {
        return withContext(viewModelScope.coroutineContext) {
            service.test()
        }
    }

which is called by a Fragment:

lifecycleScope.launch() { model.test() }

Unhelpful error:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: package_name, PID: 9729
I/Process: Sending signal. PID: 9729 SIG: 9

I'm just not sure why creating a RequestBody causes a crash. What is weird is, when debugging and going slowly through the code, it doesn't crash... I have simplified the code so its easier for you to read, but the reason why its scoped to Fragment, not VM is because I want the Fragment to change the UI based on the response.

EDIT: It turns out if I wrap it in a try/ catch block, the line which crashes usually doesn't crash. Instead, the network call afterwards does, giving printing out:

I/System.out: java.net.UnknownHostException: Unable to resolve host "qats.orth.uk": No address associated with hostname

        try {
        val requestBody = RequestBody.create(mediaType, mode.toString())
        return api.createQuiz(requestBody).createResult()
        } catch (e: Exception) {
            println(e)
        }

So I am confused. Why does putting it in a try/catch block change what error message I get. Without try/catch, I got a useless crash with no stacktrace. I guess this may be the challenge with coroutines.