[System test] : Rebalance failed with badarg error

[user:error,2018-09-11T20:27:39.219-07:00,ns_1@172.23.108.103:<0.8021.0>:ns_orchestrator:do_log_rebalance_completion:1117]Rebalance exited with reason {mover_crashed,

                              {unexpected_exit,

                               {'EXIT',<0.684.1355>,

                                {badarg,

                                 {gen_server,call,

                                  [{'janitor_agent-DISTRICT',

                                    'ns_1@172.23.108.104'},

                                   {if_rebalance,<0.17033.1318>,

                                    {dcp_takeover,'ns_1@172.23.99.21',679}},

                                   infinity]}}}}}

=========================CRASH REPORT=========================

  crasher:

    initial call: janitor_agent:-spawn_rebalance_subprocess/3-fun-0-/0

    pid: <0.18154.445>                                              

    registered_name: []

    exception error: bad argument

      in function  link/1

         called as link(undefined)                                                                                       

      in call from janitor_agent:'-handle_call/3-fun-5-'/3 (src/janitor_agent.erl, line 721)                             

      in call from janitor_agent:'-spawn_rebalance_subprocess/3-fun-0-'/3 (src/janitor_agent.erl, line 896)

    ancestors: ['janitor_agent-DISTRICT','janitor_agent_sup-DISTRICT',                                                     

                  'single_bucket_kv_sup-DISTRICT',ns_bucket_sup,                                                           

                  ns_bucket_worker_sup,ns_server_sup,ns_server_nodes_sup,

                  <0.5984.169>,ns_server_cluster_sup,<0.89.0>]                                                             

    messages: []                                                                                                           

    links: [<0.25295.403>,<0.25397.403>]

    dictionary: []                                                                                                          

    trap_exit: false                                                                                                        

    status: running

    heap_size: 987                                                                                                          

    stack_size: 27                                                                                                          

    reductions: 1004