Predictions result array empty - minimum duration?

I'm testing with an 18 second long file WAV and the predictions array is empty - is there a recommended minimum duration? Here is the avprobe output

```
Input #0, wav, from 'test.wav':
  Metadata:
    encoder         : Lavf56.1.0
  Duration: 00:00:18.25, bitrate: 256 kb/s
    Stream #0.0: Audio: pcm_s16le, 16000 Hz, 1 channels, s16, 256 kb/s
```