Cannot interpolate point data function obtained from VtkReader

I am trying to read a grid with a point-based vector field using VtkReader, and obtain the values of the vector field at the grid nodes. As the object returned by VtkReader::getPointData is a grid function I need to call Functions::interpolate with a Lagrange basis to get the actual point values. [Is that correct? The documentation doesn't mention interpolate explicitely, but I see no other way to get the point values.]

Anyway, this doesn't compile, because interpolate apparently wants to call derivative for the function returend by getPointData, which is not implemented. Here is my test program:

#include <config.h>

#include <dune/functions/functionspacebases/interpolate.hh>
#include <dune/functions/functionspacebases/lagrangebasis.hh>
#include <dune/functions/functionspacebases/powerbasis.hh>

#include <dune/grid/uggrid.hh>

#include <dune/vtk/vtkreader.hh>

using namespace Dune;

int main ()
{
  constexpr int dim = 3;
  using Grid = UGGrid<dim>;

  // Read the grid
  Vtk::VtkReader<Grid> vtkReader;
  vtkReader.read("my_file");
  auto grid = vtkReader.createGrid();

  using namespace Functions::BasisFactory;
  auto basis = makeBasis(
    grid->leafGridView(),
    power<dim>(
      lagrange<1>()
      ));

  // Read vector-valued point data
  auto displacementPointData = vtkReader.getPointData("displacement");

  // Interpolate to get the vector of coefficients
  std::vector<FieldVector<double,dim> > displacement(basis.size());
  Functions::interpolate(basis, displacement, displacementPointData);
}

Am I doing something wrong here?