Tab Container Javascript Error: Unspecified Error

I keep getting this error when I try to view a certain page with IE7. I do not get the error with Firefox 2. I have two pages that have tabs, but only receive an error on one of them. It occasionally works with no problems. If I set the first panel's visible property to false, then it works fine. The first panel contains an 2 update panels with a filtered text boxes, an update panel with a calendar extender, and an update panel with a mask edit extender and mask edit valuator. I also use framesets. Could any of this be causing the error? Does anyone know a work around?

I have found that the Masked Edit Extender is causing the error. Can I use this inside a tab panel? Is there something special I need to do to use it? If it's not possible, does anyone know of anything similar that would work in its place?