Ember FastBoot SimpleDOM body is empty

I'm trying to read from the DOM in an instance initializer in the FastBoot environment but document.body seems to be empty. The following code works in the browser but returns nothing in FastBoot:

const ELEMENT_NODE_TYPE = 1;

// TODO(maros): Get this working with FastBoot. For some reason the SimpleDOM
// body is empty.
const findBootstrapData = (document) => {
  for (let currentNode = document.body.firstChild; currentNode; currentNode = currentNode.nextSibling) {
    if (currentNode.nodeType !== ELEMENT_NODE_TYPE) {
      continue;
    }

    if (currentNode.getAttribute('class') === 'bootstrap-data') {
      return JSON.parse(currentNode.firstChild.nodeValue);
    }
  }
};

export function initialize(appInstance) {
  const store = appInstance.lookup('service:store');
  const document = appInstance.lookup('service:-document');

  const data = findBootstrapData(document);
  if (data) {
    store.push({ data: data.posts });
  }
}

export default {
  name: 'blog-posts',
  initialize
};

The data that I'm trying to read has been injected into the section using an Ember CLI addon. This works perfectly without FastBoot.

How can I get this instance initializer working in FastBoot?