Immutable struct with AA init problem

[Uranuz]

In my programme I want to make set of immutable struct objects, 
that will be initialazed at startup in shared static this() 
constructor. But using the folowing code I have compilation 
error. I think there is a problem with associative array.

For usual array we have .idup property that returns immutable 
copy of dynamic array, but for AA we have no such option. And 
there are some difficulties to create immutable AA.

But question is how to make following source compile. Do you have 
any advices?

//--------------
import std.stdio, std.conv;

struct EnumFormat
{	
protected:
	string[int] _names;
	int[] _keys;

public:
	this(string[int] names, bool isAscendingOrder = true) immutable
	{	_names = names;
		import std.algorithm;
		int[] keys = names.keys;
		
		if( isAscendingOrder )
			sort!("a < b")(keys);
		else
			sort!("a > b")(keys);
		_keys = keys;
	}
	
	string opIndex(int key)
	{	if( key in _names )
			return _names[key];
		else
			throw new Exception("Value " ~ key.to!string ~ " is not valid 
enum value!!!");
	}
	
	
	int opApply(int delegate(ref string name, ref int i) dg)
	{	foreach( key; _keys )
		{	auto result = dg(names[key], key);
			if(result)
				return result;
		}
		return 0;
	}
	
	int opApply(int delegate(ref int i) dg)
	{	foreach( key; _keys )
		{	auto result = dg(key);
			if(result)
				return result;
		}
		return 0;
	}
}

immutable(EnumFormat) magicCreatures;

//Intializing it at programme startup to make it once initialized
//and shared between all threads
shared static this()
{	
	magicCreatures = EnumFormat([1:"goblin", 2:"ork", 3:"elf", 
7:"dwarf"], false);
}


void main()
{
	foreach(name, i; magicCreatures)
	{ //Doing something
		
	}
}

//-----------------
Compilation output:
enum_test2.d(16): Error: cannot implicitly convert expression 
(names) of type string[int] to immutable(char[][int])
enum_test2.d(24): Error: cannot implicitly convert expression 
(keys) of type int[] to immutable(int[])
enum_test2.d(31): Error: cannot implicitly convert expression 
(names.dup()) of type string[int] to immutable(char[][int])
enum_test2.d(90): Error: immutable method 
enum_test2.EnumFormat.this is not callable using a mutable object
Failed: 'dmd' '-v' '-o-' 'enum_test2.d' '-I.'

How can I make this struct working with immutable constructor?