fix infinite loop in append

[willmcgugan]

Fixes #3479

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.25%. Comparing base (9c74f03) to head (f44e8bd).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3480   +/-   ##
=======================================
  Coverage   98.25%   98.25%           
=======================================
  Files          74       74           
  Lines        8072     8072           
=======================================
  Hits         7931     7931           
  Misses        141      141           

Flag	Coverage Δ
unittests	98.25% <ø> (ø)

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