Clear Select value in Angular 2

I have a select input where I have to clear the selected value. Here even If if set valueS = null Select is still showing last selected value. When I on the button to clear the selected value from Select. How to achieve it?

<select (ngModelChange)="onChange($event)" [(ngModel)]="valueS">
<option value="null" >Add filter</option>
<option value="Hi">Hi</option>
<option value="Hey">Hey</option>
<option value="Hello">Hello</option>
</select>
<button (click)="clear()">Clear</button>


in Component:

clear() {
this.valueS = null
}

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