Why exactly is void async bad?

So I understand why returning void from async would normally make no sense, but I've ran into a situation where I think it would be perfectly valid. Consider the following contrived example:

protected override void OnLoad(EventArgs e)
{
    if (CustomTask == null)
        // Do not await anything, let OnLoad return.
        PrimeCustomTask();
}
private TaskCompletionSource<int> CustomTask;

// I DO NOT care about the return value from this. So why is void bad?
private async void PrimeCustomTask()
{
    CustomTask = new TaskCompletionSource<int>();
    int result = 0;
    try
    {
        // Wait for button click to set the value, but do not block the UI.
        result = await CustomTask.Task;
    }
    catch
    {
        // Handle exceptions
    }
    CustomTask = null;

    // Show the value
    MessageBox.Show(result.ToString());
}

private void button1_Click(object sender, EventArgs e)
{
    if (CustomTask != null)
        CustomTask.SetResult(500);
}

I realize this is an unusual example, but I tried to make it simple and more generalized. Could someone explain to me why this is horrible code, and also how I could modify it to follow conventions correctly?

Thanks for any help.