Fix convert for ScalMat

[devmotion]

Fixes #211.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.37%. Comparing base (b85c8ac) to head (90fb76a).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
- Coverage   91.61%   91.37%   -0.24%     
==========================================
  Files           9        8       -1     
  Lines         680      638      -42     
==========================================
- Hits          623      583      -40     
+ Misses         57       55       -2     

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