I have bind value on option button

<select [(ngModel)]="pageInput.isValid">
  <option [value]="true">Valid</option>
  <option [value]="false">Not Valid</option>                            
</select>

2nd.html

<span [hidden]=""
*ngIf="pageInput.isValid== true" class="color">*</span>

when i comparing with this didn't get any change in my result or not showing error, if I comparing value with 'true' as a string I got result but I need value as a Boolean to the api, how can Solved this problem?? please help...