Why can't I use a user defined enumeration type (derived from uint32) as the key type for a containers.Map object?

I have derived a enumeration type from uint32. I want to use this enumeration type as keys in a containers.Map object. However, I get the following error when instantiating the map:

   Error using containers.Map
   Unsupported key specified.  See documentation for valid key types.

This puzzles me! My enumeration class is indeed numeric, so it should be casted internally. Why is this? What could be done (apart from up-casting the keys)?

Thanks in advance,

Per

[Per Appelgren]

Conversion works... But it's neither convenient nor elegant. The enumeration is typically defined like this:

    classdef hungariancomposers < uint32
        enumeration
           ZOLTAN  (0)
           FRANZ   (1)
           BELA    (2)
           GYORGY  (3)
           ERNO    (4)
        end
    end

A conversion approach that works is the following:

    keysUNCAST = { hungariancomposers.ZOLTAN hungariancomposers.GYORGY } ;
    keysCAST   = cellfun( @uint32, keysUNCAST ) ;

    values = { 'KODALY', 'LIGETI' } ;

    map = containers.Map( keysCAST, values ) ;

But... We really lose the point of enumerations by doing this. Sad!