Fix PHPStan #68

Job	Run time
Coding standard (ubuntu-latest, 8.1)	36s
Static analysis (ubuntu-latest, 8.1)	35s
Test for mutants (ubuntu-latest, 8.1)	1m 9s
Tests (ubuntu-latest, 7.4, false)	48s
Tests (ubuntu-latest, 8.0, false)	45s
Tests (ubuntu-latest, 8.1, false)	50s
Tests (macos-latest, 7.4, false)	1m 20s
Tests (macos-latest, 8.0, false)	1m 21s
Tests (macos-latest, 8.1, false)	1m 19s
Tests (windows-latest, 7.4, false)	1m 15s
Tests (windows-latest, 8.0, false)	1m 21s
Tests (windows-latest, 8.1, false)	1m 21s
Tests (ubuntu-latest, 7.4, --prefer-lowest --prefer-stable, false)	52s
Tests (ubuntu-latest, 8.2, --ignore-platform-req=php+, false)	55s
Status check - CI	3s
Code coverage finish	1s
	14m 31s

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