The "previous" button in the image viewer doesn't work

Let's take https://beta.musicbrainz.org/release/742a1ade-7364-4017-ac5d-0ef8956e05b4/cover-art as an example:

• Click the first image
• Click next
• Everything's as expected
• Click previous
• Image is still #2, text is "Image 1 of 4"
• Click next again
• Now we're back at "Image 2 of 4"

This happens on not-beta as well

This happens in Chromium 31.0.1650.63 (238485) and Firefox 26.