[Vala] can't put delegate in Gee.Map

["Colombo, Alberto" <A Colombo kingston ac uk>]

hello,

this little piece of code

   delegate Object create_func();

   class Factory : Object
   {
     Gee.Map< string, create_func?> map;
   }

fails to compile with the following error:

   Factory.vala:5.20-5.30: error: `create_func?' is not a supported generic type argument, use `?' to box 
value types
     Gee.Map< string, create_func?> map;

Using the vala 0.7.4. I think it used to work some time ago, but maybe I'm getting something completely 
wrong...

thanks
alberto

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