Fix PHPStan #326

Job	Run time
Coding standard (ubuntu-latest, 8.1)	31s
Static analysis (ubuntu-latest, 8.1)	30s
Tests (ubuntu-latest, 7.4, false)	43s
Test for mutants (ubuntu-latest, 8.1)	1m 24s
Tests (ubuntu-latest, 8.0, false)	54s
Tests (ubuntu-latest, 8.1, false)	39s
Tests (macos-latest, 7.4, false)	1m 34s
Tests (macos-latest, 8.0, false)	1m 36s
Tests (macos-latest, 8.1, false)	2m 57s
Tests (windows-latest, 7.4, false)	1m 14s
Tests (windows-latest, 8.0, false)	1m 13s
Tests (windows-latest, 8.1, false)	1m 24s
Tests (ubuntu-latest, 7.4, --prefer-lowest --prefer-stable, false)	1m 0s
Tests (ubuntu-latest, 8.2, --ignore-platform-req=php+, false)	52s
Status check - CI	1s
Code coverage finish (ubuntu-latest, 8.1)	3s
	16m 35s

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