ngStyle problem

component.template.html

<!-- work -->
<div class="wave" [style.height.px]="step.DoneItemCount/step.TotalItemCount*150"></div>
<!-- not work -->
<div class="wave" [ngStyle]="{height:step.DoneItemCount/step.TotalItemCount*150}"></div>

step.data.ts

import { Item } from './item.data';
import { StateType } from './stateType.enum';

export class Step {
    Id: number;
    Title: string;
    ItemList: Item[];
    SelectedFlag: boolean;

    get TotalItemCount() {
        return this.ItemList.length;
    }

    get DoneItemCount() {
        return this.ItemList.filter(p => p.State === StateType.Done || p.State === StateType.DelayDone).length;
    }
}