#77 error generating tool paths - stl included

[Pinnacom]

An unexpected exception occoured: please send the text below to the developers of PyCAM. Thanks a lot!
Traceback (most recent call last):
File "/home/robert/pycam-0.5.1/src/pycam/Gui/Project.py", line 4041, in generate_toolpath
self.model, toolpath_settings, callback=draw_callback)
File "/home/robert/pycam-0.5.1/src/pycam/Toolpath/Generator.py", line 62, in generate_toolpath_from_settings
backend, callback)
File "/home/robert/pycam-0.5.1/src/pycam/Toolpath/Generator.py", line 372, in generate_toolpath
maxz, dz, callback)
File "/home/robert/pycam-0.5.1/src/pycam/PathGenerators/ContourFollow.py", line 258, in GenerateToolPath
draw_callback, progress_counter, num_of_triangles)
File "/home/robert/pycam-0.5.1/src/pycam/PathGenerators/ContourFollow.py", line 267, in GenerateToolPathSlice
z, progress_counter=progress_counter)
File "/home/robert/pycam-0.5.1/src/pycam/PathGenerators/ContourFollow.py", line 310, in get_potential_contour_lines
for result, ignore_triangle_id_list in results_iter:
File "/home/robert/pycam-0.5.1/src/pycam/Utils/threading.py", line 649, in run_in_parallel_local
for result in imap_func(func, args):
File "/usr/lib/python2.7/multiprocessing/pool.py", line 653, in next
raise value
AttributeError: 'NoneType' object has no attribute 'sub'