"Couldn't resolve host" error when using "I am logged in as..."

When I try to invoke a step 'I am logged in as "admin"' (for example). I get this error:
[curl] 6: Couldn't resolve host 'default' [url] http://default/usr/bin//user/reset/1/1393204481/6k6qAOWcQTaJ1KhKM8emnfRi...

This can be fixed by changing the call to the url function in generateOneTimeLogin to include a base_url parameter:

public function generateOneTimeLogin($account) {
$timestamp = time();
return url("/user/reset/$account->uid/$timestamp/" . user_pass_rehash($account->pass, $timestamp, $account->login), array('absolute' => TRUE, 'base_url' => $this->base_url));
}

I've created a property on the class called $base_url here, but I imagine I should be able to get the base_url from behat.yml.
Looks like a bug, but perhaps I am missing some configuration somewhere...