Improve code coverage

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 53.33333% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 95.68%. Comparing base (f03d0a7) to head (b15d16c).

Files	Patch %	Lines
src/C_API.jl	53.33%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #103 +/- ## ========================================== + Coverage 94.60% 95.68% +1.08% ========================================== Files 3 3 Lines 463 464 +1 ========================================== + Hits 438 444 +6 + Misses 25 20 -5 ```

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