Flexible collision library - octree - how to consider unknown cells as occupied cells?

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The objective is to create map of the environment using a quadcopter but at a given point the quadcopter should be able to "go home" based on the current known environment. The quadcopter is using a lidar and the environment is encoded in an octomap (nothing new here). To plan the path between the current pose and the home pose (i.e from where te qheadcopter took off) I am using OMPL + FCL (for checking collision between a binary octomap and a box - containing the quadcopter).

The problem I am facing, is that FCL considers "unknown cells" as "free cells" (sorry I connot post a pic because I do not have enough points) and thus the computed trajectory is going through these "unknown cells". But the idea is to plan a path within the free cells only.

The question is how to tell to fcl that the "unknown cells" must be considered as "occupied cells"?

I am aware that few people are using FCL (and it is hard to find examples) so the question will pobably be unanswered but if somebody knows some forums, it will be great as well

Thank you