I am trying to connect to LinkedIn using LWP::Authen::OAuth2. Authorisation goes fine but when I come to exchange the authorisation token for an access token, I get this error:

Endpoint: https://api.linkedin.com/v2/accessToken
JSON:
{
  "serviceErrorCode":65604,
  "message":"Empty oauth2 access token",
  "status":401
}
[download]

That doesn't seem to make alot of sense to me as I would expect the OAuth2 Access Token to be empty in a request to get it!

This is the bare bones of what I am doing...

my $linkedin = LWP::Authen::OAuth2->new(
    client_id                => 'xxxxxxx',
    client_secret            => 'xxxxxxx',
    authorization_endpoint   => 'https://api.linkedin.com/uas/oauth2/a
+uthorization',
    token_endpoint           => 'https://api.linkedin.com/v2/accessTok
+en',
    redirect_uri             => "https://$ENV{'HTTP_HOST'}/cgi-bin/pos
+tdog.pl?command=authorize_linkedin",
    scope                    => 'w_member_social',
    save_tokens              => \&save_linkedin_token,
);

#########################
# LinkedIn button clicked
sub linkedin {
    my $auth_url = $linkedin->authorization_url;
    print "Location: $auth_url\n\n";
    exit 0;
}
[download]

The code above behaves as expected by going off to LinkedIn, authorising the app and calling the callback URL.
The callback URL does this:

sub authorize_linkedin {
    my $token = $linkedin->request_tokens(
        code  => $data{'code'},
    );
    print "Content-type: text-plain\n\n";
    print "ERROR: $data{'error'}\n\nMessage: $data{'error_description'
+}\n\n";
    print "TOKEN: $token\n";
    print $data{'code'};
    exit 0;
}
[download]

The error (above) is generated at the request_tokens call. $data{'code'} contains the code passed as a query parameter to the callback URL.

I feel I must be missing something obvious here...