diff --git a/src/MinCoverage/MinCoverageResult.php b/src/MinCoverage/MinCoverageResult.php
index aa3f0d8..bfe6bb2 100644
--- a/src/MinCoverage/MinCoverageResult.php
+++ b/src/MinCoverage/MinCoverageResult.php
@@ -61,7 +61,7 @@ public function exitOnLowCoverage(): bool
     public static function mapFromRulesAndMetrics(
         MinCoverageRules $minCoverageRules,
         array $metrics,
-        CoverageMetric $metricTotal = null,
+        ?CoverageMetric $metricTotal = null,
     ): array {
         $results = [];
         foreach ($minCoverageRules->getRules() as $minCoverageRule) {