VMTK Module - extract center line makes the model invisible / erased!

So many times (almost always) when I use the “extract center line” function of the VMTK module, the model I used as input surface is blanked (it still appears in DATA, but invisible in any view even with visibility marked on). Why is this happening?
I am u sing the follow workaround:

  1. I save the model before extracting the center line;
  2. Then I extract the center line and my model is blanked (invisible);
  3. I load the previously saved model and everything is ok (center line + visible module).

Am I doing something wrong, or is it a problem with my CT scan?

Thank you very much again.