diff --git a/src/Illuminate/Redis/Connections/PhpRedisClusterConnection.php b/src/Illuminate/Redis/Connections/PhpRedisClusterConnection.php
index 7a9d2d0abc62..bf4816a4306e 100644
--- a/src/Illuminate/Redis/Connections/PhpRedisClusterConnection.php
+++ b/src/Illuminate/Redis/Connections/PhpRedisClusterConnection.php
@@ -13,7 +13,7 @@ public function flushdb()
     {
         $arguments = func_get_args();
 
-        $async = strtoupper((string) $arguments[0] ?? null) === 'ASYNC';
+        $async = strtoupper((string) ($arguments[0] ?? null)) === 'ASYNC';
 
         foreach ($this->client->_masters() as $master) {
             $async
diff --git a/src/Illuminate/Redis/Connections/PhpRedisConnection.php b/src/Illuminate/Redis/Connections/PhpRedisConnection.php
index acf586bf6bdc..4e68547de3d0 100644
--- a/src/Illuminate/Redis/Connections/PhpRedisConnection.php
+++ b/src/Illuminate/Redis/Connections/PhpRedisConnection.php
@@ -500,7 +500,7 @@ public function flushdb()
     {
         $arguments = func_get_args();
 
-        if (strtoupper((string) $arguments[0] ?? null) === 'ASYNC') {
+        if (strtoupper((string) ($arguments[0] ?? null)) === 'ASYNC') {
             return $this->command('flushdb', [true]);
         }
 
diff --git a/tests/Redis/RedisConnectionTest.php b/tests/Redis/RedisConnectionTest.php
index ea0667d8efe0..a89ebd2d4fc4 100644
--- a/tests/Redis/RedisConnectionTest.php
+++ b/tests/Redis/RedisConnectionTest.php
@@ -467,6 +467,28 @@ public function testItGetsMultipleKeys()
         }
     }
 
+    public function testItFlushes()
+    {
+        foreach ($this->connections() as $redis) {
+            $redis->set('name', 'Till');
+            $this->assertSame(1, $redis->exists('name'));
+
+            $redis->flushdb();
+            $this->assertSame(0, $redis->exists('name'));
+        }
+    }
+
+    public function testItFlushesAsynchronous()
+    {
+        foreach ($this->connections() as $redis) {
+            $redis->set('name', 'Till');
+            $this->assertSame(1, $redis->exists('name'));
+
+            $redis->flushdb('ASYNC');
+            $this->assertSame(0, $redis->exists('name'));
+        }
+    }
+
     public function testItRunsEval()
     {
         foreach ($this->connections() as $redis) {