Function without return does not return nil?

[Tomas Mudrunka <mudrunka@...>]

Hello,
i have callback system where user registers own function. Some of these 
functions do not return anything. I was expecting i can handle this 
condition as nil. but apparently that is not the case. any ideas how can 
i handle this situation?

> function a(); return nil; end;
> a
function: 0xaaab070bfaf0
> a()
nil
> tostring(nil)
nil
> tostring(a())
nil
> function b(); end;
> b
function: 0xaaab070c0de0
> b()
> tostring(nil)
nil
> tostring(b())
stdin:1: bad argument #1 to 'tostring' (value expected)
stack traceback:
	[C]: in function 'tostring'
	stdin:1: in main chunk
	[C]: in ?


--
S pozdravem
Best regards
     Tomáš Mudruňka - SPOJE.NET s.r.o.