Clearify serialization error message

[marc-mabe]

fixes #159

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.47%. Comparing base (415da4c) to head (cc1af53).

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Additional details and impacted files

@@            Coverage Diff            @@
##             master     #161   +/-   ##
=========================================
  Coverage     99.47%   99.47%           
  Complexity      224      224           
=========================================
  Files             4        4           
  Lines           574      574           
=========================================
  Hits            571      571           
  Misses            3        3           

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