I'm certainly no expert, but it looks good to me. Perhaps if you reversed the order of the class= and the value= bits so that value comes after class (it shouldn't make any difference, but there's nothing else in there besides ol, li, class and value -- all perfectly acceptable)?
EDIT: Could it possibly be an actual problem with EpubCheck? The version I'm using in the Calibre Editor is using version 3.3 rules vs 2.0.1 in whatever's throwing that error.
Code:
EPUBCheck 5.3.0
Validating using EPUB version 3.3 rules.