funky property error with assoc array

[Dan]

This fails to compile when accessing as m.pgoo() complaining 
about postblit.
What is wrong with this?

Note: If I alias as array instead of map: alias const(X)[] Map;
it compiles fine.
Thanks
Dan
-----------------------------
struct X { this(this) {} }
alias const(X)[string] Map;
@property int pgoo(ref Map x) { return 3; }

void main() {
  Map m;
  pgoo(m);
  m.pgoo();
}