Wrong attribute value for aria-relevant='additions removals'

When I set aria-relevant in code below equal to 'additions removals', I am getting a 'Wrong attribute value' error on the code.  

``` javascript

render () {
  return (
    <div>
      <Header location={ this.props.location } />
      <main
        role="main"
        aria-live='polite'
        aria-relevant='additions removals'
        className='clearfix'>
        { renderChildren(this.props) }
      </main>
      <Footer role="contentinfo" />
      { renderDevTools() }
    </div>
  );
}

```

Using either aria-relevant='additions' or aria-relevant='removals' works but no duplicate props are allowed.  Please help me get rid of this ugly red squiggly line under my filename.  Thanks.