doc_replicator crashes after goxdcr upgrade

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

  crasher:

    initial call: erlang:apply/2

    pid: <0.713.0>

    registered_name: xdcr_doc_replicator

    exception error: bad argument

      in operator  --/2

         called as {#Ref<0.0.8.179646>,ok} -- ['ns_1@10.111.95.104']

      in call from doc_replicator:loop/3 (src/doc_replicator.erl, line 82)

    ancestors: [<0.708.0>,ns_server_sup,ns_server_nodes_sup,<0.509.0>,

                  ns_server_cluster_sup,<0.88.0>]

    messages: []

    links: [<0.708.0>,<0.715.0>]

    dictionary: []

    trap_exit: false

    status: running

    heap_size: 1598

    stack_size: 27

    reductions: 10376