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

[Yuxuan Shui via Digitalmars-d-learn]

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.