key in aa.keys, but aa[key] give range violation?

[Yuxuan Shui via Digitalmars-d-learn]

On Wednesday, 30 March 2016 at 00:26:49 UTC, Yuxuan Shui wrote:
> My code looks something like this:
>
> bool[ulong][ulong] edge;
> foreach(u; from)
>     foreach(v; to_)
>         edge[u][v] = true;
> foreach(u; edge.keys) {
>     auto adj = edge[u];
>     //....
> }
>
> And sometimes edge[u] would give Range violation error.

I guess I'm not supposed to do aa.remove() while iterate through 
it?