title: KeyError when loading pickled scikit-learn model using joblib
body: <p>I have an object that contains within it two <code>scikit-learn</code> models, an <code>IsolationForest</code> and a <code>RandomForestClassifier</code>, that I would like to pickle and later unpickle and use to produce predictions. Apart from the two models, the object contains a couple of <code>StandardScaler</code>s and a couple of Python lists.</p>
  
  <p>Pickling this object using <code>joblib</code> is unproblematic, but when I try to unpickle it later I get the following exception:</p>
  
  <pre><code>Traceback (most recent call last):
   File "&lt;stdin&gt;", line 1, in &lt;module&gt;
   File "/home/(...)/python3.5/site-packages/joblib/numpy_pickle.py", line 578, in load
     obj = _unpickle(fobj, filename, mmap_mode)
   File "/home/(...)/python3.5/site-packages/joblib/numpy_pickle.py", line 508, in _unpickle
     obj = unpickler.load()
   File "/usr/lib/python3.5/pickle.py", line 1039, in load
     dispatch[key[0]](self)
  KeyError: 0
  </code></pre>
  
  <p>The same application both pickles and unpickles the object, so the versions of <code>scikit-learn</code>, <code>joblib</code> and other libraries are the same. I'm not sure where to start debugging, given the vague error. Any ideas or pointers?</p>