Generating fonts from Designspace failed: unsigned short is greater than maximum [error when generating variable font]

```
INFO:fontTools.varLib:Merging TT hinting
DEBUG:fontTools.ttLib.ttFont:Compiling 'glyf' table
DEBUG:fontTools.ttLib.ttFont:Writing 'glyf' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'maxp' table
DEBUG:fontTools.ttLib.ttFont:Writing 'maxp' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'loca' table
DEBUG:fontTools.ttLib.ttFont:Writing 'loca' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'head' table
DEBUG:fontTools.ttLib.ttFont:Writing 'head' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'hmtx' table
DEBUG:fontTools.ttLib.ttFont:Writing 'hmtx' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'hhea' table
DEBUG:fontTools.ttLib.ttFont:Writing 'hhea' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'OS/2' table
DEBUG:fontTools.ttLib.ttFont:Writing 'OS/2' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'cmap' table
DEBUG:fontTools.ttLib.ttFont:Writing 'cmap' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'name' table
DEBUG:fontTools.ttLib.ttFont:Writing 'name' table to disk
DEBUG:fontTools.ttLib.ttFont:Compiling 'post' table
ERROR:root:In 'SHSans-VF-ufo-test.designspace': Generating fonts from Designspace failed: unsigned short is greater than maximum
Traceback (most recent call last):
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontmake\font_project.py", line 1003, in run_from_designspace
self._run_from_designspace_interpolatable(
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontmake\font_project.py", line 1086, in _run_from_designspace_interpolatable
self.build_variable_font(
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontmake\font_project.py", line 342, in build_variable_font
font = ufo2ft.compileVariableTTF(
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\ufo2ft\__init__.py", line 570, in compileVariableTTF
return call_postprocessor(varfont, baseUfo, glyphSet=None, **kwargs)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\ufo2ft\__init__.py", line 74, in call_postprocessor
postProcessor = postProcessorClass(otf, ufo, glyphSet=glyphSet)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\ufo2ft\postProcessor.py", line 50, in __init__
self.otf = _reloadFont(otf)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\ufo2ft\postProcessor.py", line 394, in _reloadFont
font.save(stream)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\ttFont.py", line 184, in save
writer_reordersTables = self._save(tmp)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\ttFont.py", line 224, in _save
self._writeTable(tag, writer, done, tableCache)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\ttFont.py", line 647, in _writeTable
tabledata = self.getTableData(tag)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\ttFont.py", line 669, in getTableData
return self.tables[tag].compile(self)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\tables\_p_o_s_t.py", line 51, in compile
data = data + self.encode_format_2_0(ttFont)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontTools\ttLib\tables\_p_o_s_t.py", line 174, in encode_format_2_0
indices.append(index)
OverflowError: unsigned short is greater than maximum

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

Traceback (most recent call last):
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontmake\__main__.py", line 602, in main
project.run_from_designspace(inputs.designspace_path, **args)
File "C:\Users\User\AppData\Local\Programs\Python\Python310\lib\site-packages\fontmake\font_project.py", line 1017, in run_from_designspace
raise FontmakeError(
fontmake.errors.FontmakeError: In 'SHSans-VF-ufo-test.designspace': Generating fonts from Designspace failed: unsigned short is greater than maximum
```

How should I begin with debugging the issue? The above comes from generating fonts from two UFOs and a DesignSpace file.