Ion select not showing

Ionic Framework
1

hi ,
when i used ion select in ionic 4 like this

> <ion-item>
>     <ion-label>Gender</ion-label>
>     <ion-select placeholder="Select One">
>       <ion-select-option value="f">Female</ion-select-option>
>       <ion-select-option value="m">Male</ion-select-option>
>     </ion-select>
>   </ion-item>

select box now shing and page block and can’t use page
why ??!!