I spoke to a GSI Rep. a few months ago about this issue, he told me that they make two sizes of cups, one that fits over the Nalgene and one that is just under sized. I am not sure why but before you buy one make certain it is the larger one. I went to the same store two different time a couple months apart,est time they had the small one, the next time they had the one that fit.

Hope this helps, I know for a fact if you order it from campmor it will fit.

goat