How to toggle class to body tag in Vuejs?

How I can toggle class for body tag, I have tried couple of ways to resolve my problem, but still recieve error message and have no ideas for it. So, this is my story:

1. On body tag, I added v-bind:class

<body id="app" v-bind:class='{active:showModal}'>

And then I recieved these errors:

[Vue warn]: v-bind:class="{active:showModal}": attribute interpolation is not allowed in Vue.js directives and special attributes.
[Vue warn]: Invalid expression. Generated function body:  scope.active:scope.showModal
[Vue warn]: Error when evaluating expression "active:showModal". Turn on debug mode to see stack trace.

2. Then I tried to resolve my problem with these changes on body tag:

<body id="app" @v-bind:class='active:showModal'>

But I still have one error:

[Vue warn]: Invalid expression. Generated function body:  scope.active:scope.showModal

How I can resolve that one?