Error when using Ikfast translation3D with MoveIt : "Can only compute FK for Transform6D IK type!"

Hi, I'm willing to plan for a 3dof leg with moveit, using an ikfast transform3D plugin, but it fails with the error "Can only compute FK for Transform6D IK type!". The IK is OK (tested in rviz with the moveit plugin). I use setPositionTarget, and was thinking that orientation wasn't used. For now, i've tweaked computeFK() in ikfast_moveit_plugin.cpp to return a dummy orientation, but 1) it's ugly and 2) the planning often do not behave correctly, indicating that the fix isn't correct. What do i miss ? Context : debian stretch / Ros melodic