ar_track_alvar PoseStamped header has no frame

edit

I am using ar_track_alvar to detect AR tags. Even though the message type AlvarMarker is defined to have a PoseStamped field:

Header header
uint32 id
uint32 confidence
geometry_msgs/PoseStamped pose

the header frame of pose is empty:

(Pdb) !ar_marker_message.pose
header:
  seq: 0
  stamp:
    secs: 0
    nsecs:         0
  frame_id: ''
pose:
  position:
    x: -0.233206003331
    y: 0.0684338694868
    z: 1.09863466712
  orientation:
    x: 0.707971473462
    y: 0.699653287423
    z: -0.0962229695998
    w: -0.00167639034365

This is totally unexpected and confusing. Spent a lot of time debugging a tf issue resulted from this. Why would the frame not be set if the field type is PoseStamped?

(I know this may not have an answer, but just want to put it here in case someone runs into a similar problem with ar_track_alvar. Also, I am not posting on their github repo's issue as it is not responsive there).