tf invalid argument

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Hi, I'm trying to follow this tutorial, and am running into an error. The error is

    [ERROR] [1497197044.658735156]: Exception thrown while processing service call: Invalid argument passed to lookupTransform argument target_frame in tf2 frame_ids cannot be empty
[ERROR] [1497197044.663532989]: Service call failed: service [/localize_part] responded with an error: Invalid argument passed to lookupTransform argument target_frame in tf2 frame_ids cannot be empty

The code I am running is:

 bool localizePart(myworkcell_core::LocalizePart::Request& req,
                       myworkcell_core::LocalizePart::Response& res)
     {
        // Read last message
           fake_ar_publisher::ARMarkerConstPtr p = last_msg_;
           if (!p) return false;

           res.pose = p->pose.pose;
           tf::Transform cam_to_target;
           tf::poseMsgToTF(p->pose.pose, cam_to_target);

           tf::StampedTransform req_to_cam;

           listener_.lookupTransform(req.base_frame, p->header.frame_id, ros::Time(0), req_to_cam);

           req_to_target = req_to_cam * cam_to_target;
           tf::poseTFToMsg(req_to_target, res.pose);
           return true;
     }

From what I've seen, this error implies one of my arguments in the lookupTransforms function is not initialized. I'm not sure how to fix this, any help will be appreciated.

    /*
 * vision_node.cpp
 *
 *  Created on: May 9, 2017
 *      Author: donni
 */
/**
 * Simple ROS node
 */

#include <ros/ros.h>
#include <fake_ar_publisher/ARMarker.h>
#include <myworkcell_core/LocalizePart.h>
#include <tf/transform_listener.h>

class Localizer
{
public:
//Here we are defining the variables which are used in the Localizer class
    ros::ServiceServer server_;
    ros::Subscriber ar_sub_;
    fake_ar_publisher::ARMarkerConstPtr last_msg_;
    tf::TransformListener listener_;


    tf::Transform req_to_target;

    Localizer(ros::NodeHandle& nh) // This is the constructor
    {
        ar_sub_ = nh.subscribe<fake_ar_publisher::ARMarker>("ar_pose_marker", 1,
        &Localizer::visionCallback, this);
        server_ = nh.advertiseService("localize_part", &Localizer::localizePart, this);
    }

    void visionCallback(const fake_ar_publisher::ARMarkerConstPtr& msg)
    {
        last_msg_ = msg;
       // ROS_INFO_STREAM(last_msg_->pose.pose);
    }

    bool localizePart(myworkcell_core::LocalizePart::Request& req,
                       myworkcell_core::LocalizePart::Response& res)
     {
        // Read last message
           fake_ar_publisher::ARMarkerConstPtr p = last_msg_;
           if (!p) return false;

           res.pose = p->pose.pose;
           tf::Transform cam_to_target;
           tf::poseMsgToTF(p->pose.pose, cam_to_target);

           tf::StampedTransform req_to_cam;

           listener_.lookupTransform(req.base_frame, p->header.frame_id, ros::Time(0), req_to_cam);

           req_to_target = req_to_cam * cam_to_target;
           tf::poseTFToMsg(req_to_target, res.pose);
           return true;
     }

};


int main(int argc, char* argv[])
{
       // This must be called before anything else ROS-related
       ros::init(argc, argv, "vision_node");

       // Create a ROS node handle
       ros::NodeHandle nh;

       ROS_INFO("Hello, World!");

       // The Localizer class provides this node's ROS interfaces
          Localizer localizer(nh);


          ROS_INFO("Vision node starting");

       // Don't exit the program.
       ros::spin();
}