diff --git a/src/KeyedFileMutex.php b/src/KeyedFileMutex.php
index d2cb6d2..da646f8 100644
--- a/src/KeyedFileMutex.php
+++ b/src/KeyedFileMutex.php
@@ -5,6 +5,7 @@
 use Amp\Sync\KeyedMutex;
 use Amp\Sync\Lock;
 use Amp\Sync\SyncException;
+use ValueError;
 use function Amp\delay;
 
 final class KeyedFileMutex implements KeyedMutex
@@ -13,21 +14,24 @@ final class KeyedFileMutex implements KeyedMutex
 
     private readonly Filesystem $filesystem;
 
+    private readonly string $directory;
+
     /**
-     * @param string $pattern Name of temporary file to use as a mutex, including an %s to splice the key in
+     * @param string $directory Directory in which to store key files.
      */
-    public function __construct(private readonly string $pattern, ?Filesystem $filesystem = null)
+    public function __construct(string $directory, ?Filesystem $filesystem = null)
     {
-        if (!\preg_match("((%%(*SKIP))*%s)", $this->pattern)) {
-            throw new \Error("Invalid pattern for a mutex, needs to contain an unescaped %s");
-        }
-
         $this->filesystem = $filesystem ?? filesystem();
+        $this->directory = \rtrim($directory, "/\\");
+
+        if (!$this->filesystem->isDirectory($this->directory)) {
+            throw new ValueError(\sprintf('Directory "%s" does not exist', $this->directory));
+        }
     }
 
     public function acquire(string $key): Lock
     {
-        $filename = \sprintf($this->pattern, \hash('sha256', $key));
+        $filename = $this->getFilename($key);
 
         // Try to create the lock file. If the file already exists, someone else
         // has the lock, so set an asynchronous timer and try again.
@@ -63,4 +67,9 @@ private function release(string $filename): void
             );
         }
     }
+
+    private function getFilename(string $key): string
+    {
+        return $this->directory . '/' . \hash('sha256', $key) . '.lock';
+    }
 }
diff --git a/test/KeyedFileMutexTest.php b/test/KeyedFileMutexTest.php
index 2a4cbe0..2852721 100644
--- a/test/KeyedFileMutexTest.php
+++ b/test/KeyedFileMutexTest.php
@@ -8,8 +8,20 @@
 
 final class KeyedFileMutexTest extends AbstractKeyedMutexTest
 {
+    protected function setUp(): void
+    {
+        parent::setUp();
+        Fixture::init();
+    }
+
+    protected function tearDown(): void
+    {
+        parent::tearDown();
+        Fixture::clear();
+    }
+
     public function createMutex(): KeyedMutex
     {
-        return new KeyedFileMutex(\sys_get_temp_dir() . '/testmutex-' . \bin2hex(\random_bytes(5)) . '-%s.lock');
+        return new KeyedFileMutex(Fixture::path());
     }
 }