Robot_localization diagnostics topic question

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I'm currently attempting to fine-tune an instance of (ekf) robot_localization using gps and single imu devices. In doing so, the diagnostic topic lists:

- 
    level: 1
    name: ekf_localization: Filter diagnostic updater
    message: Potentially erroneous data or settings detected for a robot_localization state estimation node.
    hardware_id: none
    values: 
      - 
        key: imu0_pose_covariance
        value: The covariance at position (35), which corresponds to YAW variance, was zero. This will be replaced with a small value to maintain filter stability, but should be corrected at the message origin node.
  -

The location of “position (35)” YAW covariance eludes me in attempting to revise the value. I've searched all the code for robot_localization and the code for the Phidgets 1044 imu to no avail. The ekf_localization launch file initial covariance is set (currently at a non-zero value of 0.035). The "message origin node" is the elusive part.

Aside from the ekf_localization initial_estimate_covariance, is there anywhere left to look for modification?

Thanks in advance for any insight.

B2256