As you can see here (https://github.com/jquery/jquery/blob/master/src/ajax.js#L628), $.ajax has special handling for HTTP 204 responses thus that no attempt is made to convert their body (because as per the RFC, 204 responses MUST NOT have a body).

No such special casing is done for 205 RESET CONTENT responses, which also MUST NOT have a body, thus that 205 responses received with a Content-Type that should be converted fail unexpectedly (their fail() handler is called instead of done()).

IMHO, the trivial and obvious fix is to add a similar special case for 205 responses. Since I'm not a regular contributor, I doubt a pull request is worth the extra for to produce one.

Also, see here (http://jsfiddle.net/Sj3a6/2/) for a small jsfiddle+heroku app combo that demonstrates the problem.