GNSS Heading Updates API call failure

[GavPL]

We are running 3DMGX5-AHRS device and attempting to remove the magnetometer data from the EKF calculations. We initialise the EKF heading value from an external GNSS.
The initialisation appears to work okay and I can continue as previously.
When I attempt to to pass further headings from external GNSS to IMU I get an API call failure (retCode 4) but I cannot work out error is indicated.

Code Snippet

if (!data.ekfInitialised_) {
SetImuIdle();
data.node_.setInitialHeading(ostkmath::degreesToRadians(nmeaHDTData.heading));
    data.ekfInitialised_ = true;
StartImuSampling();
} else {
      mscl::HeadingData headingData;
      headingData.headingAngle = ostkmath::degreesToRadians(nmeaHDTData.heading); // pass radians value to function
      // SetImuIdle();   
      data.node_.sendExternalHeadingUpdate(headingData); // THIS CALL FAILS
      // StartImuSampling();
}
}