Fix PHPStan #75

Job	Run time
Coding standard (ubuntu-latest, 8.1)	31s
Test for mutants (ubuntu-latest, 8.1)	35s
Static analysis (ubuntu-latest, 8.1)	34s
Tests (ubuntu-latest, 7.4, false)	39s
Tests (ubuntu-latest, 8.0, false)	49s
Tests (ubuntu-latest, 8.1, false)	50s
Tests (macos-latest, 7.4, false)	1m 38s
Tests (macos-latest, 8.0, false)	1m 37s
Tests (macos-latest, 8.1, false)	2m 0s
Tests (windows-latest, 7.4, false)	1m 11s
Tests (windows-latest, 8.0, false)	1m 12s
Tests (windows-latest, 8.1, false)	1m 11s
Tests (ubuntu-latest, 7.4, --prefer-lowest --prefer-stable, false)	38s
Tests (ubuntu-latest, 8.2, --ignore-platform-req=php+, false)	49s
Status check - CI	2s
Code coverage finish (ubuntu-latest, 8.1)	3s
	14m 19s

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