External rapidjson?

Since fides seems to allow it, I’ve tried to build with -DFIDES_USE_EXTERNAL_RAPIDJSON=ON. However it fails as such:

CMake Error at CMake/vtkModule.cmake:3530 (add_library):
  Target "fides" links to target "RapidJSON::RapidJSON" but the target was
  not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?
Call Stack (most recent call first):
  ThirdParty/fides/vtkfides/fides/CMakeLists.txt:42 (vtk_module_add_module)

Not sure what the issue is exactly, this is beyond my knowledge of CMake.