How to save Octomap binary and full probabilities files with octomap_server own's tools?

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Hi all,

I have been trying for several hours to save octomap files. First I fired up octomap_server_node and have supplied the node with PointCloud 2. The binary and full probability streams are outputted flawlessly. But when I followed the instructions as written in octomap_server octomap_saver and type the command line :

$ rosrun octomap_server octomap_saver testmap.bt

It does save a file, but when I try to run the file with:

$ rosrun octomap_server octomap_server_node testmap.bt

It returns with the messages:

[ INFO] [1489994026.637410422]: Publishing latched (single publish will take longer, all topics are prepared)
[ WARN] [1489994026.656581542]: Nothing to publish, octree is empty
[ INFO] [1489994026.708117222]: Octomap file test_map.bt loaded (502627 nodes).

I really don't understand why all .bt and .ot files that I managed to save so far have the message Nothing to publish, octree is empty every time I am playing them back. And hence, what is the correct or proper way of saving an octomap? Thank you !