Can't read from AudioRecord when encoding is PCM-Float

No matter what I do, I can't get the Android AudioRecord class to work with Pcm-Float.

Here is my code:

public AndroidAudioDriver(Sensor sensor, int desiredSampleRate)
{
    _sensor = sensor;

    if (!InitialiseSampleRate(desiredSampleRate))
    {
        throw new NotSupportedException("No supported audio sample rates found");
    }
    recorder = new AudioRecord(AudioSource.Mic, SampleRate, ChannelIn.Mono, Encoding.PcmFloat, _bufferSize);
    _recorder.StartRecording();
    var res = _recorder.Read(_audioData, 0, (int)(SampleRate * _readPortion));
}

private bool InitialiseSampleRate(int rate)
{
    for (int i = rate; i < 48000; i += 100)
    {  // add the rates you wish to check against
        _bufferSize = AudioRecord.GetMinBufferSize(i, ChannelIn.Mono, Encoding.Pcm16bit) * 10;
        if (_bufferSize > 0)
        {
            _audioData = new byte[_bufferSize];
            SampleRate = i;
            return true;
        }
    }
    return false;
}

_recorder.read always returns -3, which according to the documentation is "ERROR_INVALID_OPERATION". The same code works just fine with Pcm-16bit. I don't know what the invalid operation is, and there aren't any errors in logcat that I can see.

Thanks.