<p>So the story is that <code>session.id</code> still works when using the memcache session store but fails when using the in memory cache which is the default - in that case you need <code>session.id.public_id</code> but that fails in memcache mode!</p>
<p>So it's working for me locally as I have memcache configured. Travis is also supposed to have it configured but apparently it isn't working...</p>

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