TF can't look up transform

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Hello, all

I wrote a tf listener,and I need the transform between /map and /goal_in_picture. I'm also wrote a node broadcasting transform from /base_link to /goal_in_picture. The rest of tf provided by navigation stack. The TF tree like this:

  `map` ----->`odom` ----->`base_link`------>`base_laser`
                                     |------>`goal_in_picture`

Here is code aiming to look up transform from /map to /goal_in_picture:

listener.waitForTransform("/goal_in_picture","/map",rospy.Time.now(),rospy.Duration(5.0))
...(wihle loop)
try:
    now = rospy.Time.now()
    listener.waitForTransform("/goal_in_picture","/map",now,rospy.Duration(1.0))
    trans,rot = listener.lookupTransform("/goal_in_picture","/map",now)
except (tf.LookupException, tf.ConnectivityException,tf.Exception):
    rospy.loginfo("tf tree error!")

but when preforming the first line , it throw a exception :

tf.Exception: Lookup would require extrapolation into past....

Also I ran tf command in terminal:

 rosrun tf tf_echo /map /goal_in_picture

It show me these message:

Failure at 1399971930.470803359
Exception thrown:Lookup would require extrapolation into the past.  Requested time 1399971930.383213043 but the earliest data is at time 1399971934.658835312, when looking up transform from frame [goal_in_picture] to frame [map]

The current list of frames is:
Frame base_link exists with parent odom.
Frame odom exists with parent map.
Frame base_laser exists with parent base_link.
Frame goal_in_picture exists with parent base_link.
...

At time 1399971934.778
- Translation: [0.803, -1.122, 0.000]
- Rotation: in Quaternion [0.000, 0.000, -0.424, 0.906]
            in RPY [0.000, 0.000, -0.876]

Any idea ? Thanks in advance .