Error when building UFO generated with Glyphs App with glyphs sets to not export

If a ufo was exported trough Glyphs App and it has some glyphs set to not export, when trying to build with fontmake it will give the following error. Am I doing something wrong or this is a bug? Thanks.

```
Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.7/bin/fontmake", line 10, in
sys.exit(main())
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/fontmake/__main__.py", line 440, in main
ufo_paths, is_instance=args.pop("masters_as_instances"), **args
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/fontmake/font_project.py", line 990, in run_from_ufos
self.build_otfs(ufos, **kwargs)
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/fontmake/font_project.py", line 197, in build_otfs
self.save_otfs(ufos, **kwargs)
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/fontTools/misc/loggingTools.py", line 374, in wrapper
return func(*args, **kwds)
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/fontmake/font_project.py", line 502, in save_otfs
self.subset_otf_from_ufo(otf_path, ufo)
File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/fontmake/font_project.py", line 569, in subset_otf_from_ufo
assert len(ufo_order) == len(ot_order)
AssertionError
```