fix batch serializers

[ydjin0602]

.bugfix: #886

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5e0e882) 94.92% compared to head (af897bd) 94.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #887      +/-   ##
==========================================
+ Coverage   94.92%   94.94%   +0.01%     
==========================================
  Files         106      106              
  Lines       16318    16356      +38     
  Branches     2615     2618       +3     
==========================================
+ Hits        15490    15529      +39     
  Misses        552      552              
+ Partials      276      275       -1     

Flag	Coverage Δ
cext	91.63% <100.00%> (+0.01%)	⬆️
integration	94.59% <100.00%> (+0.01%)	⬆️
purepy	94.43% <100.00%> (+0.01%)	⬆️
unit	44.82% <11.11%> (-0.09%)	⬇️

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