diff --git a/src/FileMutex.php b/src/FileMutex.php
index 0881bdf..d4b4392 100644
--- a/src/FileMutex.php
+++ b/src/FileMutex.php
@@ -7,6 +7,7 @@
 use Amp\Sync\Mutex;
 use Amp\Sync\SyncException;
 use function Amp\delay;
+use const Amp\Process\IS_WINDOWS;
 
 final class FileMutex implements Mutex
 {
@@ -39,7 +40,7 @@ public function acquire(?Cancellation $cancellation = null): Lock
         // so set an asynchronous timer and try again.
         for ($attempt = 0; true; ++$attempt) {
             try {
-                $file = $this->filesystem->openFile($this->fileName, 'c');
+                $file = $this->filesystem->openFile($this->fileName, 'a');
                 if ($file->lock(LockMode::Exclusive)) {
                     return new Lock(fn () => $this->release($file));
                 }