Is this NEXT behavior expected or lucky coincidence?

[<tonyp@...>]

Hi all,

I have been using the following iterator for some time but it just occurred 
to me that it may work by lucky accident.

I have tested it with Lua v5.1, v5.2, and v5.3 and it always gives correct 
results for any table that contains any combination of key-value pairs 
and/or lists (indexed 1..n) provided there are no gaps in the 1..n numeric 
sequence:

------------------
function kpairs(t)
 local key = #t
 if key == 0 then key = nil end
 return next,t,key
end
------------------

"for k,v in kpairs(t) do print(k,v) end" will iterate over and print all 
non-list (1..n) elements of the given table.  (Again, assuming there are no 
gaps in the sequence.)
But this behavior does not seem to be guaranteed by the 'next' documentation 
(unless I missed something) which says: "The order in which the indices are 
enumerated is not specified, even for numeric indices."

(BTW: In LuaJIT, it fails just for index 0 -- and possibly other cases I 
didn't test as I rarely use LuaJIT)

So, my question is:
Can I rely on this behavior for standard Lua, or is this an implementation 
accident that could stop working any time a new version comes out?
(Or, is it even the case that even the current implementation won't work for 
all cases, and I simply was lucky enough not to hit those?  Examples where 
it wouldn't work?)

TIA