Imagedump Item Parameter Not Working

[pyromartian]

I gave an image dump command >imagedump 500 | items=20 and it said

Downloading all images/gifs/webms from the last 499 messages in channel art
Saving to image_dump/2018_06_05T00_10_27_art Check console for progress.
Parameters: Up to 20 items. 

and I assumed it would only download the first 20 images but it downloaded what it could from 499 posts.
Done! 455 items downloaded. 101.2 MB Finished in: 185.35 seconds.