POST Request not working properly in Yii2

I am using Yii2 basic application. I am using REST API. I have a studentrecords table containing Id,FirstName,LastName,Class columns. Here Id is Primary key and auto_incremented.
I have set the application configuration as required for the REST API. Below is the Studentrecords Controller


namespace app\controllers;
use Yii;
use yii\rest\ActiveController;

use app\models\Studentrecords;

class StudentrecordsController extends ActiveController
    public $modelClass = 'app\models\Studentrecords';
	public function behaviors()
return [
        'class' => \yii\filters\ContentNegotiator::className(),
        'only' => ['index'],
        'formats' => [
            'application/json' => \yii\web\Response::FORMAT_JSON,

	public function actionIndex()
    \Yii::$app->response->format = \yii\web\Response::FORMAT_JSON;


I am using POSTMAN to test my API. Here GET request is working fine. But when I use POST and try to supply the values in Body, then the response I return is not proper, but the values are getting saved into tables. When I preview the tab on Postman, foll error is displayed.

# Invalid Argument – [yii\base\InvalidArgumentException](

## Response content must not be an array.

What should I do?