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> I've noticed that luaL_checkudata cannot be used with a negative stack 
> index; that is, one relative to the top of the stack. The difficulty
> is that it fetches the metatable from the registry before it fetches
> the metatable from the userdata to be checked, so the stack depth
> has changed by the time it does that.

>From the manual (

  Several functions in the auxiliary library are used to check C
  function arguments. Their names are always luaL_check* or luaL_opt*.
  [...] Because the error message is formatted for arguments (e.g., "bad
  argument #1"), you should not use these functions for other stack

-- Roberto