Problem estimating volume with octomap

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I am trying to estimate the volume of an object using octomaps, the idea is to itinerate trought all the leafs of the map and count how many occupied nodes there are and then multiplied them by the volume of each voxel, obtaining the volume of the whole object.

The problem seems to be that the octomap of the object is empty on the inside, giving me a wrong answer. Does anybody know how to correct this? Or a better metod for estimating the volume of an object?