What does the error "Failed to register a ServiceWorker: The document is in an invalid state." mean?

I'm trying to register a service worker from an iframe that is created dynamically like this:

var iframe = document.createElement('iframe');
var html = '
<head><script src="/script.js"></script></head>
<body>Iframe</body>
';
document.body.appendChild(iframe);
iframe.contentWindow.document.open();
iframe.contentWindow.document.write(html);
iframe.contentWindow.document.close();

script.js contains the code that registers a service worker (texbook example from google's documentation).

This throws an error saying "Failed to register a ServiceWorker: The document is in an invalid state."

I tried having the service worker registration code in the 'load' event handler of the iframe and adding a timeout to account for the service worker registration code potentially being called at a bad time without any success.

In the source code of Chromium I found the following code which is where I think the error is coming from:

if (!m_provider) {
        resolver->reject(DOMException::create(InvalidStateError, "Failed to register a ServiceWorker: The document is in an invalid state."));
        return promise;
    }

What does "The document is in an invalid state" mean in this context? What is m_provider and why is it that it's not available?