Fix invalid flag edge case after nargs

[brentyi]

Fixes #457, thanks @leCapi!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.64%. Comparing base (8e773a4) to head (9c06fa4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #458   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files          52       52           
  Lines        3984     3984           
=======================================
  Hits         3970     3970           
  Misses         14       14           

Flag	Coverage Δ
unittests	99.64% <ø> (ø)

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