title: PolynomialFeatures fit_transform is giving Value error
body: <p>I am getting a ValueError while trying to run the Polynomial Regression example:</p>
  
  <pre><code>from sklearn.preprocessing import PolynomialFeatures
  import numpy as np
  
  poly = PolynomialFeatures(degree=2)
  poly.fit_transform(X)   ==&gt; ERROR
  </code></pre>
  
  <p>The error is:</p>
  
  <pre><code>File "/root/.local/lib/python2.7/site-packages/sklearn/base.py", line 426, in fit_transform
      return self.fit(X, **fit_params).transform(X)
  
  File "/root/.local/lib/python2.7/site-packages/sklearn/preprocessing/data.py", line 473, in fit
    self.include_bias)
  
  File "/root/.local/lib/python2.7/site-packages/sklearn/preprocessing/data.py", line 463, in _power_matrix
    powers = np.vstack(np.bincount(c, minlength=n_features) for c in combn)
  
  File "/usr/lib/python2.7/dist-packages/numpy/core/shape_base.py", line 226, in vstack
    return _nx.concatenate(map(atleast_2d,tup),0)
  
  File "/root/.local/lib/python2.7/site-packages/sklearn/preprocessing/data.py", line 463, in &lt;genexpr&gt;
  
    powers = np.vstack(np.bincount(c, minlength=n_features) for c in combn)  
    ValueError: The first argument cannot be empty.
  </code></pre>
  
  <p>My scikit-learn version is 0.15.2</p>
  
  <p>This is example is taken from: <a href="http://scikit-learn.org/stable/modules/linear_model.html#polynomial-regression-extending-linear-models-with-basis-functions" rel="nofollow">http://scikit-learn.org/stable/modules/linear_model.html#polynomial-regression-extending-linear-models-with-basis-functions</a></p>