LUA script add item to end of data gives stack overflow

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I'm writing my first dissector, so have a noob question, I'm afraid.

How do I add an item from a fixed offset to the end of the data. According to the docs -1 is mean to do this, as in:

local t_body = t_pao:add(buffer(35,-1), "Body")

but when I try that, or range(), I get:

Lua Error: ...ds\WiresharkPortable64-development\Data\plugins\pao.lua:43: C stack overflow