Uncaught exception when shutting down server

[do3cc]

Copy&Pasted from zodb/relstorage#14
written by @neilferreira

Hi all

I'm getting an error when shutting down my server in my development environment.
Plone 4.3.3
RelStorage = 1.5.1

sauna and get the same issue when CTRL + C'ing the insance.

[instance]
<= instance_base
recipe = plone.recipe.zope2instance
http-address = 8080
blob-dir ${buildout:var-dir}/blobstorage
dsn dbname = ${buildout:database-name} host=${buildout:database-host} user=${buildout:database-user}

^C2014-06-17 14:15:21 INFO SignalHandler Caught signal SIGINT
2014-06-17 14:15:21 INFO Z2 Shutting down
Traceback (most recent call last):
  File "/home/neilf/development/website/website-website.wa.gov.au-v2/parts/instance/bin/interpreter", line 328, in 
    exec(compile(file__f.read(), __file, "exec"))
  File "/home/neilf/development/website/buildout-cache/eggs/Zope2-2.13.22-py2.7.egg/Zope2/Startup/run.py", line 76, in 
    run()
  File "/home/neilf/development/website/buildout-cache/eggs/Zope2-2.13.22-py2.7.egg/Zope2/Startup/run.py", line 26, in run
    starter.run()
  File "/home/neilf/development/website/buildout-cache/eggs/Zope2-2.13.22-py2.7.egg/Zope2/Startup/init.py", line 108, in run
    self.shutdown()
  File "/home/neilf/development/website/buildout-cache/eggs/Zope2-2.13.22-py2.7.egg/Zope2/Startup/init.py", line 113, in shutdown
    db.close()
  File "/home/neilf/development/website/buildout-cache/eggs/ZODB3-3.10.5-py2.7-linux-x86_64.egg/ZODB/DB.py", line 624, in close
    @self._connectionMap
  File "/home/neilf/development/website/buildout-cache/eggs/ZODB3-3.10.5-py2.7-linux-x86_64.egg/ZODB/DB.py", line 506, in _connectionMap
    self.pool.map(f)
  File "/home/neilf/development/website/buildout-cache/eggs/ZODB3-3.10.5-py2.7-linux-x86_64.egg/ZODB/DB.py", line 206, in map
    self.all.map(f)
  File "/home/neilf/development/website/buildout-cache/eggs/transaction-1.1.1-py2.7.egg/transaction/weakset.py", line 58, in map
    f(elt)
  File "/home/neilf/development/website/buildout-cache/eggs/ZODB3-3.10.5-py2.7-linux-x86_64.egg/ZODB/DB.py", line 628, in _
    c._release_resources()
  File "/home/neilf/development/website/buildout-cache/eggs/ZODB3-3.10.5-py2.7-linux-x86_64.egg/ZODB/Connection.py", line 1075, in _release_resources
    c._storage.release()
AttributeError: 'NoneType' object has no attribute 'release'