What happens when you try to serialize a closure

[Finn Wilcox <finnw@...>]

I just noticed this question on stackoverflow
(http://stackoverflow.com/questions/14509814)

The poster was trying to serialize a closure (using string.dump.)
This is their code sample:

> function ffactory(x) return function() return x end end
> f1 = ffactory(5)
> print(f1())
5                        <-- so far so good
> s = string.dump(f1)
> f2 = load(s)
> print(f2())
table: 00000000002F7BA0  <-- expected the integer 5
> print(f2()==_ENV)
true                     <-- definitely didn't expect this!

I understand that f2 has no way to access the original upvalue (which is
why one should not pass a function with upvalues to string.dump.)

But still, I agree with the "definitely didn't expect this!" remark.
Shouldn't f2() return nil rather than _ENV?
Is this a bug?