When a non-existing page is visited (in an allowed namespace) the Wikispeech toolbar is loaded. Upon attempting to play the "There is currently no text in this page." text a TypeError: utterance is undefined is raised. It used to be possible to play this text back when we synthesized the rendered output.
The quick short-term fix is to Disable the toolbar on non-existing pages.
A long term fix is suggested in T255112: Synthezise default error messages