ionic4 切換卡 ion-segment-button 默認不顯示

 <div class="question"></div>
       <ion-segment [(ngModel)]="button" >
	        <ion-segment-button value="overdue">
	          
	          <span clas="iconSize">借閱排行</span>
	        </ion-segment-button>
	        <ion-segment-button value="resolve" >
	          <span clas="iconSize">閱讀排行</span>
	        </ion-segment-button>
	   </ion-segment>     
    </div>