Using two "heterogeneous" subscribers

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Hi all, I am using ROS Noetic. My goal is to use two subscribers, one to get bounding boxes from Yolo and the other is to get the image from the camera. For that, I am using message_filters, specifically, the ApproximateTimeSynchronizer policy. The problem is that the callback does not work. The program is the following:

#!/usr/bin/env python3.6
import rospy
import math
import numpy as np

import roslib
import sys 


import message_filters
from cv_bridge import CvBridge, CvBridgeError
from std_msgs.msg import Header
from sensor_msgs.msg import Image, CameraInfo
import matplotlib.image as mpimg
import cv2
import glob
from perspective.msg import BoundingBoxes, BoundingBox 

class listener():
  def __init__(self):
      print("camera 1")
      self.bridge= CvBridge()
      self.image_sub = message_filters.Subscriber('/CAM_0/image_raw', Image)
      self.image_bounding_sub= message_filters.Subscriber('/darknet_ros/bounding_boxes', BoundingBoxes)

      self.ts = message_filters.ApproximateTimeSynchronizer([ self.image_bounding_sub,  self.image_sub], queue_size=100, slop=0.1, allow_headerless=True)
      self.ts.registerCallback(self.callback)
      #rospy.spin()
      print("camera 2")

  def callback(self,data,imageraw):
      print("callback")     
      for i in range(0,len(data.bounding_boxes)-1): 
          #print("id", data.bounding_boxes[i].id)
          #print("xmax", data.bounding_boxes[i].xmax)                           
          if data.bounding_boxes[i].xmax < 1100 and data.bounding_boxes[i].xmax > 177:
             print("In ROI", data.bounding_boxes[i].xmax)          
          cv_image = self.bridge.imgmsg_to_cv2(imageraw,"bgr8")


def main():  
            rospy.init_node('perspective', anonymous=True)
            #sum()
            listener()
            print("main")
            rospy.spin()

if __name__ == '__main__':
      print("start")
       main()

When I make the two subscribers with the same type (Bounding Box or camera), it works. Does anyone, please, have an idea to fix this problem? thank you.