message: Fix typo in exception.
diff: @@ -537,7 +537,7 @@ class Layer(object):
  if x.get_shape().ndims is None:
  raise ValueError('Input ' + str(input_index) + ' of layer ' +
  self.name + ' is incompatible with the layer: '
  - 'its rank is undefined, by the layer requires a '
  + 'its rank is undefined, but the layer requires a '
  'defined rank.')
  # Check ndim.
  