fonttools: ttx dies on file with no glyphs

I created a new file in Glyphs, turned off exporting for all characters, and exported as OTF. Then I fed it to ttx:

$ ttx TTXTest-Regular.otf
Dumping "TTXTest-Regular.otf" to "TTXTest-Regular.ttx"...
Dumping 'GlyphOrder' table...
Dumping 'head' table...
Dumping 'hhea' table...
Dumping 'maxp' table...
Dumping 'OS/2' table...
Dumping 'name' table...
Dumping 'cmap' table...
Dumping 'post' table...
Dumping 'CFF ' table...
.notdef
A
ERROR: Unhandled exception has occurred
Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/ttx.py", line 383, in main
    process(jobs, options)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/ttx.py", line 356, in process
    action(input, output, options)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/misc/loggingTools.py", line 372, in wrapper
    return func(*args, **kwds)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/ttx.py", line 256, in ttDump
    bitmapGlyphDataFormat=options.bitmapGlyphDataFormat)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/ttLib/__init__.py", line 302, in saveXML
    self._tableToXML(tableWriter, tag, progress)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/ttLib/__init__.py", line 338, in _tableToXML
    table.toXML(writer, self, progress)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/ttLib/tables/C_F_F_.py", line 42, in toXML
    self.cff.toXML(writer, progress)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/cffLib.py", line 86, in toXML
    font.toXML(xmlWriter, progress)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/cffLib.py", line 1549, in toXML
    BaseDict.toXML(self, xmlWriter, progress)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/cffLib.py", line 1508, in toXML
    conv.xmlWrite(xmlWriter, name, value, progress)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/cffLib.py", line 720, in xmlWrite
    value.toXML(xmlWriter, progress)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/cffLib.py", line 564, in toXML
    charStr, fdSelectIndex = self.getItemAndSelector(name)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/cffLib.py", line 546, in getItemAndSelector
    return self.charStringsIndex.getItemAndSelector(index)
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/cffLib.py", line 407, in getItemAndSelector
    return self[index], sel
  File "/Library/Python/2.7/site-packages/FontTools/fontTools/cffLib.py", line 337, in __getitem__
    item = self.items[index]
IndexError: list index out of range

The .notdef and A lines in there are a debugging print which I inserted into cffLib.py:

def toXML(self, xmlWriter, progress):
    names = sorted(self.keys())
    i = 0
    step = 10
    numGlyphs = len(names)
    for name in names:
        print(name)

So it sees that something in the font is claiming to have an A glyph, but there isn’t one defined. I don’t know if Glyphs is producing broken OTFs (fontlint doesn’t think so) but you’re certainly missing an index check somewhere.