fix: handle null Predis set members

src/Prometheus/Storage/RedisClients/Predis.php

@@ -83,7 +83,8 @@ public function setNx(string $key, mixed $value): void
 
     public function sMembers(string $key): array
     {
-        return $this->client->smembers($key);
+        /** @phpstan-ignore-next-line Predis can return null at runtime despite its array return type. */
+        return $this->client->smembers($key) ?? [];
     }
 
     public function hGetAll(string $key): array|false

tests/Test/Prometheus/Storage/PredisTest.php

@@ -58,4 +58,24 @@ public function itShouldNotClearWholeRedisOnFlush(): void
             self::equalTo(['not a prometheus metric key'])
         );
     }
+
+    /**
+     * @test
+     */
+    public function itShouldReturnAnEmptyArrayWhenSMembersReturnsNull(): void
+    {
+        $client = $this->getMockBuilder(Client::class)
+            ->disableOriginalConstructor()
+            ->addMethods(['smembers'])
+            ->getMock();
+
+        $client->expects(self::once())
+            ->method('smembers')
+            ->with('missing-key')
+            ->willReturn(null);
+
+        $predis = new \Prometheus\Storage\RedisClients\Predis($client);
+
+        self::assertSame([], $predis->sMembers('missing-key'));
+    }
 }