PHP Facebook api“无效的UTF-8”json_encode错误

We have a "json_encode(): Invalid UTF-8 sequence in argument" error resulting when we try to do the following:

CODE:

$facebookdetails = $facebook->api(array(
    'access_token' => $fbtoken,
    'method' => 'fql.query',
    'query' => 'SELECT uid,name,last_name,first_name,
                 FROM user where uid=me()'
        ));
echo json_encode($facebookdetails[0]);

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the facebookdetails contains this result when we print it using var_export

name' => 'ÐгÐÑÑ Ð°ÑаÑ', 'last_name' => 'ÐаÑаÑ',

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in what encoding is Facebook returning these fields? and how should this be handled to properly send it using json_encode?