diff --git a/test/Unit/RuleSet/ExplicitRuleSetTestCase.php b/test/Unit/RuleSet/ExplicitRuleSetTestCase.php
index 64b976c4..be9839cf 100644
--- a/test/Unit/RuleSet/ExplicitRuleSetTestCase.php
+++ b/test/Unit/RuleSet/ExplicitRuleSetTestCase.php
@@ -36,7 +36,7 @@ final public function testRuleSetDoesNotConfigureRuleSets(): void
         $rulesWithoutRulesForRuleSets = \array_filter(
             $rules,
             static function (string $nameOfRule): bool {
-                return '@' !== \mb_substr($nameOfRule, 0, 1);
+                return \mb_strpos($nameOfRule, '@') !== 0;
             },
             \ARRAY_FILTER_USE_KEY,
         );