Fix PrivateAttr usage

[jwodder]

Closes #1537.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.54%. Comparing base (3940aa2) to head (39a0e20).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1538      +/-   ##
==========================================
+ Coverage   88.45%   88.54%   +0.08%     
==========================================
  Files          78       78              
  Lines       10691    10691              
==========================================
+ Hits         9457     9466       +9     
+ Misses       1234     1225       -9     

Flag	Coverage Δ
unittests	88.54% <100.00%> (+0.08%)	⬆️

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[github-actions]

🚀 PR was released in 0.65.0 🚀