CI3 - Memcached obtaining lock error

I have CI 2 running on same server and using memcached no problems.

With CI3, using same memcached server and setup I'm getting the following error:

Session: Error while trying to obtain lock for ci_session:be440cbaad59f81d8a1299c9a9638db7b0fcf58a

What lock is CI trying to obtain? Is this a bug, am I missing something? I've been using memcached for years on magento, and CI 2. Seems a strange error in CI3.