IE not styling HTML5 tags (with shiv)

I'm trying to style a <header> and it's not working in IE. I'm using Modernizr, but i've tried the shiv by itself.

Example code

<section>
  <header>
    <h1>Title</h1>
  </header>
  <p>Body</p>
</section>

Styled with:

section {
    border: 2px dotted black;
    padding: 0.25em;
}

header h1 {
    background-color: #ccc;
    text-align: center;
    margin: 0;
}

I expect a border around the header and section content, but instead there is just an '[' looking border and the header is beneath it.

Whats going wrong?