Why can't I copy a const struct?

[Atila Neves]

    private struct DummyStruct {
        int[] a;

        this(this) {
            a = a.dup;
        }
    }

    void main() {
        const dummy1 = DummyStruct();
        DummyStruct dummy2 = dummy1;
    }

    struct_copy.d(12): Error: conversion error from 
const(DummyStruct) to DummyStruct

Surely I should be able to copy it and use the mutable version as 
I please, no? I'd understand if the postblit constructor wasn't 
defined...

Atila