在调用restful api接口时报错，请问这是什么原因啊 [ 未指定版本 ]

An Error occurred while handling another error:
exception 'yii\base\UnknownClassException' with message 'Unable to find 'api\controllers\SiteController' in file: E:\wamp\www\advanced/api/controllers/SiteController.php. Namespace missing?' in E:\wamp\www\advanced\vendor\yiisoft\yii2\BaseYii.php:296
Stack trace:
#0 [internal function]: yii\BaseYii::autoload('api\controllers...')
#1 [internal function]: spl_autoload_call('api\controllers...')
#2 E:\wamp\www\advanced\vendor\yiisoft\yii2\base\Module.php(637): class_exists('api\controllers...')
#3 E:\wamp\www\advanced\vendor\yiisoft\yii2\base\Module.php(596): yii\base\Module->createControllerByID('site')
#4 E:\wamp\www\advanced\vendor\yiisoft\yii2\base\Module.php(522): yii\base\Module->createController('site/error')
#5 E:\wamp\www\advanced\vendor\yiisoft\yii2\web\ErrorHandler.php(108): yii\base\Module->runAction('site/error')
#6 E:\wamp\www\advanced\vendor\yiisoft\yii2\base\ErrorHandler.php(111): yii\web\ErrorHandler->renderException(Object(yii\web\NotFoundHttpException))
#7 [internal function]: yii\base\ErrorHandler->handleException(Object(yii\web\NotFoundHttpException))
#8 {main}
Previous exception:
exception 'yii\web\NotFoundHttpException' with message 'Page not found.' in E:\wamp\www\advanced\vendor\yiisoft\yii2\web\Request.php:284
Stack trace:
#0 E:\wamp\www\advanced\vendor\yiisoft\yii2\web\Application.php(82): yii\web\Request->resolve()
#1 E:\wamp\www\advanced\vendor\yiisoft\yii2\base\Application.php(386): yii\web\Application->handleRequest(Object(yii\web\Request))
#2 E:\wamp\www\advanced\api\web\index.php(17): yii\base\Application->run()
#3 {main}