How to add class to angular component in storybook?

Question

I defined a component "test-component-wrapper" with viewencapsulation setted to none. Its template has only an

<ng-content></ng-content>> 

I defined also a "test-component-item" with this template

<div>Some text</div>

In storybook I tried to create a story with this template

<test-component-wrapper>
 <test-component-item class="testClass">
 </test-component-item>
</test-component-wrapper>

But it doesn't work...dom is not rendered. If I add this story without class it works. I tried so to it directly in angular project and class is added. How can I solve?