Angular: conditional class with *ngClass

What is wrong with my Angular code? I am getting the following error:

Cannot read property 'remove' of undefined at BrowserDomAdapter.removeClass

<ol><li *ngClass="{active: step==='step1'}" (click)="step='step1'">Step1</li><li *ngClass="{active: step==='step2'}" (click)="step='step2'">Step2</li><li *ngClass="{active: step==='step3'}" (click)="step='step3'">Step3</li></ol>