diff --git a/vfs_lock_test.go b/vfs_lock_test.go
new file mode 100644
index 0000000..afac3d3
--- /dev/null
+++ b/vfs_lock_test.go
@@ -0,0 +1,80 @@
+package sqlite3
+
+import (
+	"context"
+	"os"
+	"runtime"
+	"testing"
+)
+
+func Test_vfsLock(t *testing.T) {
+	switch runtime.GOOS {
+	case "linux", "darwin":
+		//
+	default:
+		t.Skip()
+	}
+
+	file1, err := os.CreateTemp("", "sqlite3-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer file1.Close()
+
+	name := file1.Name()
+	defer os.RemoveAll(name)
+
+	file2, err := os.OpenFile(name, os.O_RDWR, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer file2.Close()
+
+	vfsOpenFiles = append(vfsOpenFiles, &vfsOpenFile{
+		file:      file1,
+		nref:      1,
+		vfsLocker: &vfsFileLocker{file1, _NO_LOCK},
+	}, &vfsOpenFile{
+		file:      file2,
+		nref:      1,
+		vfsLocker: &vfsFileLocker{file2, _NO_LOCK},
+	})
+
+	mem := newMemory(128)
+	mem.writeUint32(4+4, 0)
+	mem.writeUint32(16+4, 1)
+
+	rc := vfsCheckReservedLock(context.TODO(), mem.mod, 4, 32)
+	if rc != _OK {
+		t.Fatal("returned", rc)
+	}
+	if got := mem.readUint32(16); got != 0 {
+		t.Error("file was locked")
+	}
+
+	rc = vfsLock(context.TODO(), mem.mod, 16, _SHARED_LOCK)
+	if rc != _OK {
+		t.Fatal("returned", rc)
+	}
+
+	rc = vfsCheckReservedLock(context.TODO(), mem.mod, 4, 32)
+	if rc != _OK {
+		t.Fatal("returned", rc)
+	}
+	if got := mem.readUint32(32); got != 0 {
+		t.Error("file was locked")
+	}
+
+	rc = vfsLock(context.TODO(), mem.mod, 16, _EXCLUSIVE_LOCK)
+	if rc != _OK {
+		t.Fatal("returned", rc)
+	}
+
+	rc = vfsCheckReservedLock(context.TODO(), mem.mod, 4, 32)
+	if rc != _OK {
+		t.Fatal("returned", rc)
+	}
+	if got := mem.readUint32(32); got == 0 {
+		t.Error("file wasn't locked")
+	}
+}
diff --git a/vfs_unix.go b/vfs_unix.go
index 614ecc2..c770e17 100644
--- a/vfs_unix.go
+++ b/vfs_unix.go
@@ -156,7 +156,7 @@ func (l *vfsFileLocker) CheckReservedLock() (bool, xErrorCode) {
 	if l.fcntlGetLock(&lock) != nil {
 		return false, IOERR_CHECKRESERVEDLOCK
 	}
-	return lock.Type == syscall.F_UNLCK, _OK
+	return lock.Type != syscall.F_UNLCK, _OK
 }
 
 func (l *vfsFileLocker) fcntlGetLock(lock *syscall.Flock_t) error {