FileWatcher Mutex - Object synchronization method was called from an unsynchronized block of code.

public async Task OnModifiedAsync(FileSystemEventArgs e)
{
    if (e.FullPath.ToLower().EndsWith(extension_to_read))
    {
        string text = await CalculateSha256Async(e.FullPath);
        if (file_hash != text)
        {
            file_hash = text;
            Log.Information("File " + e.FullPath + " has been modified.");
            Log.Information("Watching folder: " + folder);
            await importer.DoImporterActionAsync(e.FullPath);
        }
    }
}

public async void OnModifiedAsyncHandler(object sender, FileSystemEventArgs e)
{
    try
    {
        mutex.WaitOne();
        await OnModifiedAsync(e);
    }
    finally
    {
        mutex.ReleaseMutex();
    }
}