500 Internal Server Error when trying to view per-server download times

Steps to reproduce:

1. Open https://metrics.torproject.org/torperf.html
2. Select "Source: moria"
3. Click "Update Graph" Or just follow link: https://metrics.torproject.org/torperf.png?start=2017-12-12&end=2018-03-12&source=moria&server=public&filesize=50kb

Result: no graph image (because of error).

Trac:
Username: Vort