In #479 we moved from iscntrl -> !isgraph. The problem there is non-ascii characters are "not graphical", but they are valid keys according to the memcached server.

I understand the libmemcached client does a !isgraph, and thus it may be worth adopting this (even though protocol does technically support non-ascii keys), but at a minimum this should probably be a major release as unexpected loss of keys is a really scary unexpected change.