Cannot idup an Associative array

[thewilsonator]

void main() {
    int[int] aa1 = [1:2];
    immutable aa2 = aa1.dup;
}

fails with

onlineapp.d(3): Error: none of the overloads of template `object.idup` are callable using argument types `!()(int[int])`
/dlang/dmd/linux/bin64/../../src/druntime/import/object.d(3871):        Candidates are: `idup(T)(T[] a)`
/dlang/dmd/linux/bin64/../../src/druntime/import/object.d(3880):                        `idup(T : void)(const(T)[] a)`