To recreate:

 client = Mongo::Client.new(["[::1]:27017"])
 client.with(heartbeat_frequency: 20)
 NoMethodError: undefined method `[]' for nil:NilClass
  from mongo-ruby-driver/lib/mongo/address/ipv6.rb:51:in `parse'

Reason is that the IPv6 class expects the address to be formatted with brackets but a ipv6 address object does not include the brackets when the host attribute is accessed. See Cluster#initialize when each seed is added.