Converting json object to dataframe-character '0' not recognized

I am trying to convert this json object to a dataframe. I am getting an error that the character "0" is not recognized. Any suggestion on how to handle this error occurring in a large json file having 1000 records?

library(RJSONIO)

json_file<- '{ "_id" : ObjectId( "539163d7bd350003" ), "login" :    "vui", "id" : 369607, "avatar_url" : "https://avatars.mashupsusercontent.com/u/369607?", "gravatar_id" : "df8897ffebe16c5b0cd690925c63e190", "url" : "https://api.mashups.com/users/vui", "html_url" : "https://mashups.com/vui", "followers_url" : "https://api.mashups.com/users/vui/followers", "following_url" : "https://api.mashups.com/users/vui/following{/other_user}", "gists_url" : "https://api.mashups.com/users/vui/gists{/gist_id}", "starred_url" : "https://api.mashups.com/users/vui/starred{/owner}{/repo}", "subscriptions_url" : "https://api.mashups.com/users/vui/subscriptions", "organizations_url" : "https://api.mashups.com/users/vui/orgs", "repos_url" : "https://api.mashups.com/users/vui/repos", "events_url" : "https://api.mashups.com/users/vui/events{/privacy}", "received_events_url" : "https://api.mashups.com/users/vui/received_events", "type" : "User", "site_admin" : false, "org" : "amurath" }'

json_file <- fromJSON(json_file)

json_file <- lapply(json_file, function(x) {
x[sapply(x, is.null)] <- NA
unlist(x)
})

df<- do.call("rbind", json_file)

### Error in fromJSON(json_file) : unexpected character 'O'