alias this and associative arrays

[Shachar Shemesh via Digitalmars-d]

import std.exception;

struct AA(Key, Val) {
    Val[Key] aa;
    alias aa this;
    void opIndexAssign(inout Val value, Key key) pure {
        aa[key] = value;
    }
}

void main() {
    AA!(string, int) a;
     //compile error -- no property 'remove' for type 
'CheckedAA!(string, int)'
    a.remove("aaa");
}

What's wrong here?