rebalance to convert tap to dcp failed when upgrade from 2.5.x to 3.1.0

Install 2.5.0 on 2 nodes (177 and 92)
Create cluster and 2 buckets, default and standard
Load 500K items to each bucket
Install 3.1.0 on other 2 nodes (195 and 199)
Do online upgrade (swap rebalance) to upgrade cluster to 3.1.0
When all nodes in cluster become 3.1.0, DCP rebalance failed with error

DCP upgrade exited with reason {{badmatch,
{error,

{failed_nodes,['ns_1@172.23.107.199']}

}},
[{dcp_upgrade,handle_call,3,
[

{file,"src/dcp_upgrade.erl"}

,

{line,65}

]},
{gen_server,handle_msg,5,
[

{file,"gen_server.erl"}

,

{line,585}

]},
{proc_lib,init_p_do_apply,3,
[

{file,"proc_lib.erl"}

,

{line,239}

]}]}
ns_orchestrator002 ns_1@172.23.107.195 13:16:10 - Fri Aug 28, 2015