Aspen like most any loose bedding comes in various size shreds and quality. In other words, not all aspen is identical. I prefer to go with a very fine shred aspen and it's working out quite nicely for us. Any tiny amount of a fine shred aspen is very unlikely to cause issues with a creature's whose gut acids can fully digest fur, bone and teeth.

Something to remember with any bedding like this is to clean down and around any wastes you find. Liquids do spread so always take a good area around the mess to make sure you aren't leaving anything nasty behind. Also with a fine bedding like aspen, it does tend to compress under the constant weight of a snake (especially inside hides). I just regularily finger fluff the bedding and it brings it's loft back up, helping to move heat from the UTH in a much more efficient and consistent manner.