Right place to put the _ERRORMESSAGE?

["jose_marin2 <jose_marin2@...>" <jose_marin2@...>]

Is there a right place to put the _ERRORMESSAGE?
My error handler lloks like this:

-----------------------------------
function l_error (s)
 
  MsgInfo(s)   
 
end

_ERRORMESSAGE = l_error

-----------------------------------

MsgInfo just displays a MessageBox.
When I put this code in the main Lua file, it works, but when it is 
defined in a file called via "dofile", it doesn't works!