What is the problem?

When using the watch/unwatch functionality on Special:Watchlist, I sometimes see:

Uncaught TypeError: title.replace is not a function
    jQuery 2
        parse
        newFromText
    <anonymous> http://dev.wiki.local.wmftest.net:8080/w/resources/src/mediawiki.special.watchlist/watchlist.js?962bc:67
    jQuery 2
        dispatch
        handle
load.php:1448:5

It seems to happen when the title of the page you are watching/unwatching is a number.

The issue appears to occur around here, because it is attempting to call the string function replace() on a number.

Steps to reproduce problem

1. As one user, edit https://test.wikipedia.org/wiki/100
2. As another user, add https://test.wikipedia.org/wiki/100 to your watchlist
3. Go to https://test.wikipedia.org/wiki/Special:Watchlist
4. Click the x next to https://test.wikipedia.org/wiki/100

Expected behavior: There should be a javascript popup saying you have unwatched the article
Observed behavior: Takes you to https://test.wikipedia.org/wiki/100?action=watch (and you see an error in the browser console)

Environment

Browser: Firefox 78.
Wiki(s): MediaWiki 1.36.0-alpha (36a0784).