Debugging Javascript SDK Proxy Auth

[LPayne91]

I'm trying to set up the Javascript SDK using proxy auth from a Rails server. Following the instructions at https://github.com/looker-open-source/sdk-codegen/tree/main/packages/sdk#using-a-proxy-for-authentication I've written a class that extends ProxySession but it's throwing an error trying to set the token:

Uncaught (in promise) Error: Cannot read properties of undefined (reading 'setToken')
    at sdkError (transport.js:171)
    at transport.js:200
    at Generator.next (<anonymous>)
    at asyncGeneratorStep (transport.js:1)
    at _next (transport.js:3)

Code:

import { ProxySession, ApiSettings, BrowserTransport } from "@looker/sdk-rtl";

class EmbedSession extends ProxySession {
  constructor(settings, transport) {
    super(settings, transport);
  }

  async authenticate(props) {
    const token = await getAuthToken();

    if (token) {
      this.activeToken.setToken(token); // <- error
    }
    // rest of this method is copy-pasted from the example, I don't even know if it works yet
  }
}

Stepping through the code it looks like this.activeToken object is undefined, and I'm not sure why. Other than this EmbedSession I haven't really touched the ApiSettings and BrowserTransport passed to initialise the SDK (using const LookerSDK = LookerBrowserSDK.init31(settings, transport, session);). Does one of those set up activeToken?

The actual token should be fine, it's coming from a LookerSDK::Client instance in the Rails backend, using method #access_token. That SDK auths and hits Looker just fine, with surprisingly little setup.