AA with immutable values

[Roman D. Boiko]

immutable(SomeClass)[S] _map;
_map[S.init] = null; // doesn't compile since _map[S.init] is 
immutable

My current workaround is:
immutable(SomeClass)*[S] _map;
_map[S.init] = [null].ptr;

But this introduces an additional level of indirection in a 
performance-critical part of my code.

Probably I could cast away immutable before storing, and cast 
back to immutable after retrieving. But I would be more happy 
without these two casts. (By the way, is it correct that this way 
would be more efficient?)

Is there any other way to put immutable values into a dictionary?