Fix custom error message for union type

[justin-tay]

Closes #1137

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.09%. Comparing base (48ca3c2) to head (e596c08).
Report is 73 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1138      +/-   ##
============================================
- Coverage     78.90%   78.09%   -0.81%     
- Complexity     1965     2058      +93     
============================================
  Files           172      201      +29     
  Lines          6352     6849     +497     
  Branches       1255     1251       -4     
============================================
+ Hits           5012     5349     +337     
- Misses          867     1033     +166     
+ Partials        473      467       -6     

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