Fix corner cases of combine_selection.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.99%. Comparing base (e9492c1) to head (86cee41). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1038 +/- ## ========================================== + Coverage 86.88% 86.99% +0.10% ========================================== Files 101 101 Lines 6008 6058 +50 ========================================== + Hits 5220 5270 +50 Misses 788 788 ```

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