Getting error for Facebook SDK login

Here’s the code which is invoked upon successful Facebook login:

func userLoggedIn() {
// now, log in as Backendless user
let mappings = [
"id" : "facebookId",
"name" : "name",
"birthday": "birthday",
"first_name": "fb_first_name",
"last_name" : "fb_last_name",
"gender": "gender",
"email": "email"
]
let backendless = Backendless.sharedInstance()!
let token = FBSDKAccessToken.current()!
backendless.userService.login(withFacebookSDK: token,
fieldsMapping: mappings,
response: { (backendlessUser) in
print("successfully logged in using facebook \(backendlessUser)")
// check if that's a new user
},
error: { (error) in
print("error login in using facebook \(error)")
let loginManager = LoginManager()
loginManager.logOut()
})
}

error handler is invoked, however:
error login in using facebook Optional(FAULT = ‘404’ [HttpEngine: INVALID statusCode 404] <HttpEngine: INVALID statusCode 404> )
what am I missing out here?

figured out the problem - I was passing wrong string as the appVersion to

backendless?.initApp(APP_ID, secret: SECRET_KEY, version: BACKEND_VERSION)

in AppDelegate