Can't get data from Spotify api - node.js

Question

I want to get the data of artists followed by a user, but when I make the request, it just gives me an error code 400, and the message 'Only valid bearer authentication supported'. I'm not sure what's wrong. Here's the code I'm using.
app.get('/callback', function(req, res) {
var code = req.query.code || null;
var state = req.query.state || null;
var storedState = req.cookies ? req.cookies[stateKey] : null;
if (state === null || state !== storedState) {
res.redirect('/#' +
querystring.stringify({
error: 'state_mismatch'
}));
} else {
res.clearCookie(stateKey);
var authOptions = {
url: 'https://accounts.spotify.com/api/token',
form: {
code: code,
redirect_uri: redirect_uri,
grant_type: 'authorization_code'
},
headers: {
'Authorization': 'Basic ' + (new Buffer(client_id + ':' + client_secret).toString('base64'))
},
json: true
};
request.post(authOptions, function(error, response, body) {
if (!error && response.statusCode === 200) {
var access_token = body.access_token,
refresh_token = body.refresh_token;
var options = {
url: 'https://api.spotify.com/v1/me/following?type=artist&limit=50',
headers: { 'Authorization': 'Bearer' + access_token },
json: true
};
request.get(options, function(error, response, body) {
console.log(body);
});
res.redirect('/#' +
querystring.stringify({
access_token: access_token,
refresh_token: refresh_token
}));
} else {
res.redirect('/#' +
querystring.stringify({
error: 'invalid_token'
}));
}
});
}
});