This document: http://sequel.rubyforge.org/rdoc-plugins/classes/Sequel/Plugins/Caching.html

...lacks valuable information. cache_store must also implement delete(), i.e. delete( key, time=nil ).

Reference: http://deveiate.org/code/Ruby-MemCache/classes/MemCache.html

I agree. I've fixed it in my private branch, it should be pushed to github soon.