title: yii2:radioList must be a string
body: <p>I have this field in my _form.php</p>
  
  <pre><code>&lt;?= $form-&gt;field($model, 'delivered')-&gt;radioList
  ([ 'delivered' =&gt; 'Delivered', 'part' =&gt; 'Partly Delivered']) ?&gt;
  </code></pre>
  
  <p>I am getting the error, when I am selecting the radio-box Delivered I am getting the error - </p>
  
  <p><strong>delivered must be a string</strong></p>
  
  <p>I can't make out what is wrong in the syntax.</p>
  
  <p>Validation rule in the model is like:</p>
  
  <pre><code>public function rules()
      {
          return [
              [['ipd_patient_id', 'room_name'], 'integer'],
              [['request_time', 'issue_time'], 'safe'],
              [['general_regn_no'], 'string', 'max' =&gt; 15],
              [['patient_name'], 'string', 'max' =&gt; 50],
              [['delivered'], 'string', 'max' =&gt; 20]
          ];
      }
  </code></pre>
  
  <p>Thanks.</p>