Memcached’s EINVAL essentially indicates a bug with the client (in this case, libcouchbase) as it should filter and validate the input. There is an actual abort() call and it seems to be a debugging leftover - although do note that receiving an EINVAL is quite severe.
This bug was introduced in 2.5.6 of libcouchbase, so it’s fairly recent.
https://issues.couchbase.com/browse/CCBC-689
However I would be interested in seeing what sort of input caused the EINVAL