Segfault when casting array of Interface types

[rcor via Digitalmars-d-learn]

I'm back for another round of "is this a bug, or am I doing 
something stupid?".

C and D implement interface I, and I have an array of each. I'd 
like to combine these into one I[], but eventually I'd like to 
cast an element back to its original type.

interface I {}
class C : I {}
class D : I {}

void main() {
  C[] c = [new C, new C];
  D[] d = [new D, new D];
  auto i = cast(I[]) c ~ cast(I[]) d;
  assert(cast(C) i[0]); // segfault
}

casting each array to I[] is fine, but casting an element back to 
C segfaults (even if it weren't a C, it should just return null, 
right?)