Fix issue 308

[khdesai]

No description provided.

[codecov-io]

Codecov Report

Merging #315 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
+ Coverage   98.15%   98.15%   +<.01%     
==========================================
  Files         123      123              
  Lines       13894    13913      +19     
==========================================
+ Hits        13638    13657      +19     
  Misses        256      256

Impacted Files	Coverage Δ
stix2/v21/sdo.py	95.4% <ø> (ø)	⬆️
stix2/test/v21/test_indicator.py	100% <100%> (ø)	⬆️
stix2/v20/bundle.py	100% <100%> (ø)	⬆️
stix2/v21/bundle.py	100% <100%> (ø)	⬆️
stix2/v20/sdo.py	100% <100%> (ø)	⬆️
stix2/properties.py	98.4% <100%> (+0.01%)	⬆️
stix2/base.py	98.84% <100%> (ø)	⬆️

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[clenk]

Looks good to me. Thanks @khdesai!