Using array as parameter in service

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I am using array as input and output in a service. I am trying to extend the existing tutorial) for getting started. Below is the client code-

#include "ros/ros.h"
#include "beginner_tutorials/AddTwoInts.h"
#include <cstdlib>

int main(int argc, char **argv) {
  ros::init(argc, argv, "add_two_array_client");

  ros::NodeHandle n;
  ros::ServiceClient client =
      n.serviceClient<beginner_tutorials::AddTwoInts>("add_two_array");
  beginner_tutorials::AddTwoInts srv;

  std::vector<double> a(3);
  std::vector<double> b(3);

  a.push_back(1.0);
  a.push_back(2.0);
  a.push_back(3.0);

  b.push_back(4.0);
  b.push_back(5.0);
  b.push_back(6.0);

  srv.request.a = a;
  srv.request.b = b;
  if (client.call(srv)) {

    for (int i = 0; i < 3; i++) {
      ROS_INFO("Sum: %f", (float)srv.response.sum[i]);
    }
  } else {
    ROS_ERROR("Failed to call service add_two_array");
    return 1;
  }

  return 0;
}

Following is the server code-

#include "ros/ros.h"
#include "beginner_tutorials/AddTwoInts.h"

bool add(beginner_tutorials::AddTwoInts::Request &req,
         beginner_tutorials::AddTwoInts::Response &res) {
  res.sum.resize(3);
  for (int i = 0; i < 3; i++) {
    res.sum[i] = req.a[i] + req.b[i];
    ROS_INFO("request: x=%f, y=%f", req.a.at(i), req.b[i]);
    ROS_INFO("sending back response: [%f]", res.sum[i]);
  }
  return true;
}

int main(int argc, char **argv) {
  ros::init(argc, argv, "add_two_array_server");
  ros::NodeHandle n;

  ros::ServiceServer service = n.advertiseService("add_two_array", add);
  ROS_INFO("Ready to add two ints.");
  ros::spin();

  return 0;
}

The srv file looks like following-

float64[] a
float64[] b
---
float64[] sum

While running, the input parameters shows 0 value. Below is the snippet from terminal-

[ INFO] [1465212136.691882517, 8990.505000000]: request: x=0.000000, y=0.000000
[ INFO] [1465212136.691913058, 8990.505000000]: sending back response: [0.000000]

How to use array as parameter in service?