CMAP decompile error

[moi15moi]

When I am trying to get an font cmap, I get an exception
You can download the font here: DFHSMincho-W9__DFPHSMincho-W9__DFGHSMincho-W9.zip

Sample

from fontTools.ttLib.ttFont import TTFont

ttFont = TTFont("DFHSMincho-W9__DFPHSMincho-W9__DFGHSMincho-W9.TTC", fontNumber=0)
cmap_table = ttFont["cmap"].getcmap(3,1)

print(cmap_table.cmap)

Which raise this exception:

Traceback (most recent call last):
  File "c:\Users\Admin\Desktop\import ctypes.py", line 6, in <module>
    print(cmap_table.cmap)
  File "C:\Users\Admin\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\tables\_c_m_a_p.py", line 289, in __getattr__
    self.ensureDecompiled()
  File "C:\Users\Admin\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\tables\_c_m_a_p.py", line 278, in ensureDecompiled
    self.decompile(None, None)  # use saved data.
  File "C:\Users\Admin\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\tables\_c_m_a_p.py", line 918, in decompile
    assert index < lenGIArray, (
AssertionError: In format 4 cmap, range (165), the calculated index (22351) into the glyph index array is not less than the length of the array (22351) !

Why is it happening?
What can I do to avoid it.

Possible solution (line 923 of fontTools\ttLib\tables\_c_m_a_p.py) - Add an verification if the index is part of the glyphIndexArray:

if index < lenGIArray and glyphIndexArray[index] != 0:  # if not missing glyph
    glyphID = glyphIndexArray[index] + delta
else:
    glyphID = 0  # missing glyph

[justvanrossum]

The font is invalid; ots also rejects it.

[moi15moi]

I tested with libass which use freetype and it can correctly decode the cmap. GDI and DirectWrite also decode it correctly.
Why fonttools should reject it?