`sum_per_block` doesn't work

• Open can.ex2, all variables, apply and go to last timestep.
• apply Python calculator as follows:

It fails as follows:

Traceback (most recent call last):
  File "<string>", line 4, in <module>
  File "/home/utkarsh/Kitware/ParaView3/ParaViewBin-Qt5/lib/site-packages/paraview/calculator.py", line 205, in execute
    output.GetAttributes(retVal.Association).append(\
  File "/home/utkarsh/Kitware/ParaView3/ParaViewBin-Qt5/lib/site-packages/vtk/numpy_interface/dataset_adapter.py", line 964, in GetAttributes
    return CompositeDataSetAttributes(self, type)
  File "/home/utkarsh/Kitware/ParaView3/ParaViewBin-Qt5/lib/site-packages/vtk/numpy_interface/dataset_adapter.py", line 768, in __init__
    self.__determine_arraynames()
  File "/home/utkarsh/Kitware/ParaView3/ParaViewBin-Qt5/lib/site-packages/vtk/numpy_interface/dataset_adapter.py", line 774, in __determine_arraynames
    dsa = dataset.GetAttributes(self.Association)
  File "/home/utkarsh/Kitware/ParaView3/ParaViewBin-Qt5/lib/site-packages/vtk/numpy_interface/dataset_adapter.py", line 904, in GetAttributes
    return DataSetAttributes(self.VTKObject.GetAttributes(type), self, type)
TypeError: GetAttributes argument 1: an integer is required (got type NoneType)