Message filter headerless warning with custom message type

edit

I generated my custom message as following:

uint32 num_object
nav_msgs/Odometry custom

This is a part of the output when echoing a topic using my custom message:

---
num_object: 1
custom:
    header:
        seq: 0
        stamp:
            secs: 1636331652
            nsecs: 425379753
        frame_id: "base_footprint"
     child_frame_id: 'base_link'
    pose:
        pose:
          position:
.......

Here, the only part I'm not updating is a seq value. I provide the stamps using rospy.Time.now()

I'm using ApproximateTimeSynchronizer to synchronize the custom message with another nav_msgs/Odometry message which has a header information:

pose_sub = message_filters.Subscriber('/something', Pose)
odom_sub = message_filters.Subscriber('/odom', Odometry)
ts = message_filters.ApproximateTimeSynchronizer([pose_sub, odom_sub], 5, 0.2)
ts.registerCallback(self._callback)

When I run the code, it shows a following warning:

[WARN] [1636331628.465729]: Cannot use message filters with non-stamped messages. Use the 'allow_headerless' constructor option to auto-assign ROS time to headerless messages.

I'm not sure what is causing this issue even if I provided most data in the header. Does seq value also matter when using the ApproximateTimeSynchronizer?