Wikispeech should accept SSML input with empty phoneme element text node. (According to W3C:s specifications, the text node is optional.)
(The problem is that MaryTTS does not accept the empty phoneme element.)
Example of valid phoneme element with empty text node: <phoneme alphabet="x-sampa" ph="' m V N . k i"></phoneme>
Testable URLs:
Browser error message: 500 internal server error
Server error message: missing (nothing in wikispeech logs, nothing in marytts logs)