[Lua NewBie] rttlib and varfromtab: convert number to geometry_msgs.Pose

Hi all!

I'm trying to use varfromtab (from rttlib.lua) to convert a lua table to a geometry_msgs.Pose. I use the following code

    port_type = rtt.Variable(port:info().type)  -- discover port type: geometry_msgs.Pose
    value = {position={x=0.0, y=0.0, z=0.0}, orientation={w=1.0, x=0.0, y=0.0, z=0.0} }
    pcall(rttlib.varfromtab, port_type, value)
and I get the following error

    __lua_todsb: can't convert lua number to /geometry_msgs/Pose variable
Of course I'm doing something wrong, but I cannot figure out what... Thank you in advance for any help!