Reduction too large


I am using Couchbase Server 3.1.0-1776 Enterprise Edition (build-1776).
any my map function will return 33 values.
In my reduce , I want to group by result by values.group_by_key,but I got the following errors:

error (reduction too large)

I even can not return values in reduce function,such as:

function (keys, values, rereduce) {
   return values;