title: zero-size array to reduction operation maximum which has no identity
body: <p>Here is a runnable snippet of code which generates the error on my machine.
  I have this error (zero-size array to reduction operation maximum which has no identity).</p>
  
  <pre><code>reader = csv.reader(open(BASE_PATH + 'labels.csv'))
  # skip the header
  next(reader)
  
  X = []
  y = []
  
  for row in reader:
      label = row[2]
      if len(label) &gt; 0 and label.find(',') == -1:
          filename = get_filename_for_index(row[1])
          img_file = cv2.imread(BASE_PATH + filename)
          if img_file is not None:
              img_file = scipy.misc.imresize(arr=img_file, size=(120, 160, 3))
              img_arr = np.asarray(img_file)
              img_arr = apply_color_mask(img_arr)
              X.append(img_arr)
              y.append(label)
  
  
  X = np.asarray(X)
  y = np.asarray(y)
  
  encoder = LabelEncoder()
  encoder.fit(y)
  encoded_y = encoder.transform(y)
  
  y = np_utils.to_categorical(encoded_y)
  
  
  **Error**
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-7-49c49b99292b&gt; in &lt;module&gt;()
       26 encoded_y = encoder.transform(y)
       27 
  ---&gt; 28 y = np_utils.to_categorical(encoded_y)
  
  ~/.local/lib/python3.5/site-packages/keras/utils/np_utils.py in to_categorical(y, num_classes)
       20     y = np.array(y, dtype='int').ravel()
       21     if not num_classes:
  ---&gt; 22         num_classes = np.max(y) + 1
       23     n = y.shape[0]
       24     categorical = np.zeros((n, num_classes))
  
  ~/.local/lib/python3.5/site-packages/numpy/core/fromnumeric.py in amax(a, axis, out, keepdims)
     2270 
     2271     return _methods._amax(a, axis=axis,
  -&gt; 2272                           out=out, **kwargs)
     2273 
     2274 
  
  ~/.local/lib/python3.5/site-packages/numpy/core/_methods.py in _amax(a, axis, out, keepdims)
       24 # small reductions
       25 def _amax(a, axis=None, out=None, keepdims=False):
  ---&gt; 26     return umr_maximum(a, axis, None, out, keepdims)
       27 
       28 def _amin(a, axis=None, out=None, keepdims=False):
  </code></pre>
  
  <p>Value Error: zero-size array to reduction operation maximum which has no identity</p>
  
  <p>so please any one can help me.</p>