lookupUsers, userTimeline, getUser problem with "one" user only


During the analytics of a streaming api request made via twitteR I got a problem. I restricted the reason to the acquisition of one user data among the whole dataset. From a sample of 40.000+ users/tweets when I ask for info on user 514675394 (namely: “__keri16” ) I get this response:

Error in twFromJSON(out) :
Error: Malformed response from server, was not JSON.
The most likely cause of this error is Twitter returning a character which
can’t be properly parsed by R. Generally the only remedy is to wait long
enough for the offending character to disappear from searches (e.g. if
using searchTwitter()).

I tried to change encoding or search for other reasons could create any problem like ‘protected user’, but nothing I found.

Anyone can help?
Thanks in advance.