ToUniquePtr from dune-grid results in conflicting return types

foamgridfactory.hh:192:40: error: conflicting return type specified for createGrid()

dune/grid/common/gridfactory.hh:157:35: error: overriding ‘Dune::ToUniquePtr Dune::GridFactoryInterface::createGrid() [with GridType = Dune::FoamGrid<1, 2>]’ virtual ToUniquePtr createGrid() = 0;