[Paraview] Unable to use "Append attributes" filter after using "Resample with dataset" filter

[George]

Hi Paraviewers,

I would like to do a mesh refinement analysis using the predefined 
filters of paraview (I can already do such an analysis with numpy and 
pylab).
My objective is to calculate the difference between the pressure fields 
of a fine grid simulation and a coarse grid simulation.

I have two OpenFoam simulations: A fine grid simulation and a coarse 
grid simulation.
I have imported the results of both simulations into paraview.
Then I have resampled the fine grid solution as follows:
Select fine grid source >Resample with dataset > Source>Coarse grid
Then I apply a calculator on the resampled solution and basically rename 
the point pressure variable to 'presampled'
I also apply a calculator on the coarse solution and rename the point 
pressure variable to 'pcoarse'

The next step should be:
Select both calculators and apply the 'Append attributes' filter.
However, this filter is not selectable at this stage (grayed out)
I can confirm that both calculators have the same number of points and 
cells.

Can anyone tell me what I am doing wrong or whether there is a better 
way of performing a grid sensitivity study in paraview?

Thank you,
George