Facebook username field is missing

I have a server, writtien in NodeJS and in it I am requesting some information about a user on facebook given his access_token with fbgraph (version 1.1.0).

return Promise.promisify(graph.get)('me', { fields: 'picture,id,birthday,email,first_name,last_name,name,middle_name,link,timezone,updated_time,verified, ... access_token: token });

The access token is sent by the client side (android). The problem I'm running into is that for a very small percentage of requests, the response is missing a "name" field, although I get other information like email, profile picture, ...

I thought the "name" field was part of the public_profile and always provided, or can the user somehow set as hidden?