[jQuery] Increment Value. What am I doing wrong?

[shapper]

Hello,

I have an input on my page as follows:

  <input id="Index" name="Index" type="hidden" value="0" />

I am trying to increase and decrease the value of the input as
follows:

        $index = $('#Index');
        $index.val()--;

I always get an error:
invalid assignment left-hand side

What am I doing wrong?

Thanks,
Miguel