Weridness when using += or ++ with uninitialized variables

[Regnsloja]

When dealing with a previously uninitialized variables, the follow does not work properly:

variable += 1

This also does not work properly:

variable++

But the following does work:

variable = variable + 1

And this also works:

variable = 0 (somewhere during a creation step, not here right before the incrementing as that would "work" but lock it at 1 obviously)
variable += 1

Why or how I don't know, I was just trying to get someone elses simple old project to work properly and this change got a lot of stuff working in it.