Problem with Indexing a map-reduce -reduction too large-

i have a problem, when creating indexes of some maps the system go in loop.
I have seen in the logs the following error:

[mapreduce_errors:error,2016-12-19T10:21:29.397+01:00,couchdb_ns_1@<0.21371.208>:couch_set_view_util:log_port_error:813]Set view `log_ser`, main group `_design/dev_topUsage`, received error from index builder: Error building index for view `method`, reason: (view 5) reduction too large, ret = -13

Any possible solution ?
I have a bucket with 11 milion of entry and a cluster with 8 server node.
Thanks for advice

Take a look at

I have other map-reduce that do the same thing on different fields and these are working properly.
The only one that does not work is this view.
The method field is a string more long of other fields, is possible to find a solution ?