Fix PHPStan #121

Job	Run time
Coding standard (ubuntu-latest, 8.1)	38s
Static analysis (ubuntu-latest, 8.1)	44s
Test for mutants (ubuntu-latest, 8.1)	1m 6s
Tests (ubuntu-latest, 7.4, false)	47s
Tests (ubuntu-latest, 8.0, false)	36s
Tests (ubuntu-latest, 8.1, false)	40s
Tests (macos-latest, 7.4, false)	2m 8s
Tests (macos-latest, 8.0, false)	1m 31s
Tests (macos-latest, 8.1, false)	1m 32s
Tests (windows-latest, 7.4, false)	1m 17s
Tests (windows-latest, 8.0, false)	1m 2s
Tests (windows-latest, 8.1, false)	57s
Tests (ubuntu-latest, 7.4, --prefer-lowest --prefer-stable, false)	51s
Tests (ubuntu-latest, 8.2, --ignore-platform-req=php+, false)	48s
Status check - CI	7s
Code coverage finish (ubuntu-latest, 8.1)	2s
	14m 46s

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