AssertionError.

```
PS E:\workspace\Python> fontmake -u ufo2.ufo -o otf --verbose DEBUG
DEBUG:fontmake.font_project:ufoLib UFO validation is disabled
INFO:fontmake.font_project:Building OTF for MinecraftUnicode-None
INFO:ufo2ft:Pre-processing glyphs
DEBUG:ufo2ft.filters.removeOverlaps:using booleanOperations as RemoveOverlapsFilter backend
INFO:ufo2ft.filters:Running DecomposeComponentsFilter on MinecraftUnicode-Regular
INFO:ufo2ft.filters:Running RemoveOverlapsFilter on MinecraftUnicode-Regular
DEBUG:ufo2ft.filters:Took 3.316s to run RemoveOverlapsFilter on 1415 glyphs
INFO:ufo2ft:Building OpenType tables
DEBUG:ufo2ft.fontInfoData:Underline thickness not set in UFO, defaulting to UPM * 0.05
DEBUG:ufo2ft.fontInfoData:Underline position not set in UFO, defaulting to UPM * -0.075
DEBUG:ufo2ft.fontInfoData:Underline thickness not set in UFO, defaulting to UPM * 0.05
DEBUG:ufo2ft.fontInfoData:Underline position not set in UFO, defaulting to UPM * -0.075
DEBUG:ufo2ft.fontInfoData:Underline thickness not set in UFO, defaulting to UPM * 0.05
DEBUG:ufo2ft.featureWriters.kernFeatureWriter.KernFeatureWriter:No kerning data; skipped
DEBUG:ufo2ft.featureWriters.markFeatureWriter.MarkFeatureWriter:No mark-attaching anchors found; skipped
DEBUG:fontTools.ttLib.ttFont:compiling 'maxp' table
DEBUG:fontTools.ttLib.ttFont:writing 'maxp' table to disk
DEBUG:fontTools.ttLib.ttFont:compiling 'CFF ' table
ERROR:root:In 'minecraft_ufo2.ufo': Compiling UFO failed
Traceback (most recent call last):
File "c:\develop\python\lib\site-packages\fontmake\font_project.py", line 365, in _iter_compile
yield compile_func(ufo, debugFeatureFile=debugFeatureFile, **options)
File "c:\develop\python\lib\site-packages\ufo2ft\__init__.py", line 150, in compileOTF
postProcessor = PostProcessor(otf, ufo, glyphSet=glyphSet)
File "c:\develop\python\lib\site-packages\ufo2ft\postProcessor.py", line 46, in __init__
otf.save(stream)
File "c:\develop\python\lib\site-packages\fontTools\ttLib\ttFont.py", line 172, in save
writer_reordersTables = self._save(tmp)
File "c:\develop\python\lib\site-packages\fontTools\ttLib\ttFont.py", line 211, in _save
self._writeTable(tag, writer, done, tableCache)
File "c:\develop\python\lib\site-packages\fontTools\ttLib\ttFont.py", line 628, in _writeTable
self._writeTable(masterTable, writer, done, tableCache)
File "c:\develop\python\lib\site-packages\fontTools\ttLib\ttFont.py", line 632, in _writeTable
tabledata = self.getTableData(tag)
File "c:\develop\python\lib\site-packages\fontTools\ttLib\ttFont.py", line 650, in getTableData
return self.tables[tag].compile(self)
File "c:\develop\python\lib\site-packages\fontTools\ttLib\tables\C_F_F_.py", line 19, in compile
self.cff.compile(f, otFont, isCFF2=False)
File "c:\develop\python\lib\site-packages\fontTools\cffLib\__init__.py", line 192, in compile
writer.toFile(file)
File "c:\develop\python\lib\site-packages\fontTools\cffLib\__init__.py", line 418, in toFile
assert posList == lastPosList
AssertionError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "c:\develop\python\lib\site-packages\fontmake\__main__.py", line 471, in main
project.run_from_ufos(
File "c:\develop\python\lib\site-packages\fontmake\font_project.py", line 1055, in run_from_ufos
self.build_otfs(ufos, cff_version=cff_version, **kwargs)
File "c:\develop\python\lib\site-packages\fontmake\font_project.py", line 218, in build_otfs
self.save_otfs(ufos, **kwargs)
File "c:\develop\python\lib\site-packages\fontTools\misc\loggingTools.py", line 373, in wrapper
return func(*args, **kwds)
File "c:\develop\python\lib\site-packages\fontmake\font_project.py", line 490, in save_otfs
for font, ufo in zip(fonts, ufos):
File "c:\develop\python\lib\site-packages\fontmake\font_project.py", line 367, in _iter_compile
raise FontmakeError("Compiling UFO failed", ufo.path) from e
fontmake.errors.FontmakeError: In 'minecraft_ufo2.ufo': Compiling UFO failed
```
Is this a fontmake glitch? or Base file glitch?