non-constant expression while initializing two dim array

[tirithen]

How can I initialize my two dimensional array?

When I try to run the code below I get the error:

     Error: non-constant expression ["user":[cast(Capability)0], 
"administrator":[cast(Capability)1]]

Code:

     enum Capability {
       self,
       administer
     }

     alias Capabilities = immutable Capability[];

     private Capabilities[string] roleCapabilities = [
       "user" : [Capability.self], "administrator" : 
[Capability.administer]
     ];

I have tried adding more const and immutable prefixes but it 
still gives me the same error. The goal is that roleCapabilities 
should be an immutable/const two dimensional array.