merge_fonts.py AttributeError problem

[psturgeon54]

I am trying to use merge_fonts.py to merge the following:
files = [
# It's recommended to put NotoSans-Regular.ttf as the first element in the
# list to maximize the amount of meta data retained in the final merged font.
"NotoSansSC-Bold.otf",
"NotoSansTC-Bold.otf",
"NotoSansHK-Bold.otf"
]
When I run the script with Python 3.10.0 and fonttools 4.29.0, I get the log output below.
I must be doing something wrong somehow, since I guess this script has been used successfully many times before.

Merging 3 Fonts...
WARNING:fontTools.merge:Dropped cmap subtable from font '0': format 14, platformID 0, platEncID 5
WARNING:fontTools.merge:Dropped cmap subtable from font '1': format 14, platformID 0, platEncID 5
WARNING:fontTools.merge:Dropped cmap subtable from font '2': format 14, platformID 0, platEncID 5
WARNING:fontTools.merge:Dropped mapping from codepoint 0X2FB7 to glyphId 'cid44305'
WARNING:fontTools.merge:Dropped mapping from codepoint 0X2FC1 to glyphId 'cid45485'
etc.
WARNING:fontTools.merge:Dropped mapping from codepoint 0X9F9C to glyphId 'cid63096'
WARNING:fontTools.merge:Dropped mapping from codepoint 0X9F9F to glyphId 'cid47484#1'
Traceback (most recent call last):
File "C:\Work\Python\FontCoverage\merge_stuff.py", line 266, in
main()
File "C:\Work\Python\FontCoverage\merge_stuff.py", line 248, in main
font = merger.merge(valid_files)
File "C:\Work\Python\FontCoverage\venv\lib\site-packages\fontTools\merge_init_.py", line 114, in merge
table = clazz(tag).merge(self, tables)
File "C:\Work\Python\FontCoverage\venv\lib\site-packages\fontTools\merge\tables.py", line 243, in merge
private = newfont.Private # original
File "C:\Work\Python\FontCoverage\venv\lib\site-packages\fontTools\cffLib_init_.py", line 2587, in getattr
raise AttributeError(name)
AttributeError: Private