TypeError when opening a search page on a single source from a URL

Sentry Issue: ARKINDEX-FRONTEND-AP2

TypeError: this.sources.push is not a function
  at call (./js/store/corpora.js:28:8)
...
(45 additional frame(s) were not displayed)

1. Either click here, or open the search page on any Arkindex instance, uncheck all sources except one and press F5.
2. Play around with the checkboxes of each source.
3. Nothing happens, and there are TypeErrors in the console.
4. Cry.

This seems to be caused by &sources=element being a string when there is only one argument, but being an array when there are multiple (&sources=element&sources=transcription).