Error 500 on user update

I am trying to simply update a users email address. Whenever I attempt to do this, the service returns...

HTTP/1.1 500 Internal Server Error
<Error messageid="-34392527:137323c9966:-5e10">
  <reason>An internal error has occurred. Please retry your request.</reason>
</Error>

Here is my request, I am using REST and have json enabled:

PUT /rest/user/4 HTTP/1.1

Authorization: **oauth tokens**
Host: **rest server host**
Content-Length: 92
X-Forwarded-For: **IP address**
X-Target-URI: **rest server host**
Content-Type: text/plain; charset=ISO-8859-1
Connection: Keep-Alive

{"user":"4", "name":"test1", "roles":{"2": "authenticated user"}, "mail":"testing@test.com"}