IE only error: object doesn't support this property or method

IE only error: object doesn't support this property or method

When I click one of the thumbnails I'm getting the following error message on line 47 of my lightbox: "object doesn't support this property or method." It manifests itself by not showing the overlay (the opacity is set to 1 and it's black, so you should see it); nor is it handling the opacity of the close button (0.5). I'm only getting this error in IE, not in Firefox or Safari. I tried renaming the variables, thinking there might be a naming conflict, but it solved nothing.

Also, again only in IE, my background image on the "previous" button is positioned improperly.

How might I solve these two problems?

I have it in a password-protected staging area:
http://www.vicesbyproxy.com/staging/cb/studio.html
username: helper
password: bacon

You'll have to endure the opening animation (3 seconds) before the thumbnail array fades up.