＆＃34; missingRequiredParameter＆＃34;与YouTube API交互时出错

Question

当我使用您的YouTube API卷曲方法（v3）时

$api_key = 'YOUR_API_KEY';
$url = 'https://www.googleapis.com/youtube/v3/channels?part=snippet,statistics&key=' . $api_key . '&forUsername=' . $_GET['username'];

// initializes the request
$curl = curl_init();
// sets the url
curl_setopt($curl, CURLOPT_URL, $url);

// enables that curl_exec() returns content instead of status code
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);

// allows redirects
curl_setopt($curl, CURLOPT_FOLLOWLOCATION, 1);

// checks if a secure site has been requested
if(preg_match('/^https:\/\//', $url)){
    // if so, verification is disabled
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, false);
}
// performs the actual request
$data = curl_exec($curl);
// destructs the request
curl_close($curl);

// this line converts the json string which is returned into a php object
$data = json_decode($data);
print_r($data);

显示

Array ( 
    [error] => Array (
        [errors] => Array (
            [0] => Array (
                [domain] => youtube.parameter 
                [reason] => missingRequiredParameter 
                [message] => No filter selected. 
                [locationType] => parameter 
                [location] => 
            ) 
        )
        [code] => 400 
        [message] => No filter selected. 
    ) 
)

为什么会出现这个错误？