fovis_ros error

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Hi,

I'm working with turtlebot and kinect (hydro) in ubuntu 12.04. I integrated the visual odometry (fovis_ros). So I launched turtlebot_bringup minimal.launch to get odom and imu topics and launched openni to get the depth camera topic and fovis_hydro_openni.launch:

<launch>
  <arg name="camera" default="camera" />   
  <node pkg="nodelet" type="nodelet" args="manager" name="nodelet_manager" />   
  <node pkg="nodelet" type="nodelet" 
        name="convert_openni_fovis" 
        args="load depth_image_proc/convert_metric nodelet_manager">
    <remap from="image_raw" to="$(arg camera)/depth_registered/sw_registered/image_rect_raw"/>
    <remap from="image" to="$(arg camera)/depth/image_rect"/>   
  </node>  

  <node pkg="fovis_ros" type="fovis_mono_depth_odometer" name="kinect_odometer" >
    <remap from="/camera/rgb/image_rect" to="$(arg camera)/rgb/image_rect_mono" />
    <remap from="/camera/rgb/camera_info" to="$(arg camera)/rgb/camera_info" />
    <remap from="/camera/depth_registered/camera_info" 
           to="$(arg camera)/depth_registered/sw_registered/camera_info" />
    <remap from="/camera/depth_registered/image_rect" to="$(arg camera)/depth/image_rect" />
    <param name="approximate_sync" type="bool" value="True" />   
  </node>
</launch>

and finally i launched the robot_pose_ekf.launch which fuse the three topics (odom,imu,and vo) to publish the odom_combined topic. But i had this error:

[ERROR] [1431006366.365384721]: Covariance specified for measurement on topic vo is zero
[ERROR] [1431006366.392443095]: filter time older than vo message buffer

I think that the problem comes from the visual odometry beacause before adding visual odometry the robot_pose_ekf works fine it fused odom and imu data and published odom_combined

Any help please