diff --git a/tests/Service/Rtt/RttClosingBalanceServiceTest.php b/tests/Service/Rtt/RttClosingBalanceServiceTest.php
index 173f904..6e0837e 100644
--- a/tests/Service/Rtt/RttClosingBalanceServiceTest.php
+++ b/tests/Service/Rtt/RttClosingBalanceServiceTest.php
@@ -80,6 +80,23 @@ final class RttClosingBalanceServiceTest extends TestCase
         );
     }
 
+    public function testCustomDeficitWeekReducesClosingBalance(): void
+    {
+        // CUSTOM (4h) : une semaine de récup +3h puis une semaine déficitaire -1h
+        // (toutes deux sans tranches 25/50). Le déficit doit réduire la clôture.
+        $recovery = new WeekRecoveryDetail(totalMinutes: 180, isFlatRecovery: true);   // +3h
+        $deficit  = new WeekRecoveryDetail(totalMinutes: -60, isFlatRecovery: true);   // -1h
+
+        $closing = $this->service()->fold(new WeekRecoveryDetail(), [$recovery, $deficit], $this->payments());
+
+        // 3h - 1h = 2h reportées, et la somme des buckets égale toujours le total.
+        self::assertSame(120, $closing->totalMinutes);
+        self::assertSame(
+            120,
+            $closing->base25Minutes + $closing->bonus25Minutes + $closing->base50Minutes + $closing->bonus50Minutes,
+        );
+    }
+
     public function testBucketSumAlwaysEqualsTotalInvariant(): void
     {
         $opening = new WeekRecoveryDetail(base25Minutes: 200, bonus25Minutes: 50, base50Minutes: 100, bonus50Minutes: 50, totalMinutes: 400);