Fix node pruning raising an AttributeError in certain cases

[PLPeeters]

Fixes #760.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.26%. Comparing base (c6e8340) to head (3f2780d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #761   +/-   ##
=======================================
  Coverage   99.26%   99.26%           
=======================================
  Files          21       21           
  Lines        3559     3559           
=======================================
  Hits         3533     3533           
  Misses         26       26           

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[adbar]

Thanks!