error using subsref and oop

[Bertrand Roessli]

Hello,

I used the new oop feature in Octave that I am trying to 
learn. 

For this function (found on the web) in the directory @date 

function [obj] = date(minute, hour, day, month, year)
% constructor saved in file date.m

obj = struct;
obj.minute = [];                % leave blank for whatever reason
obj.hour = hour;
obj.day = day;
obj.month = month;
obj.year = year;

obj = class(obj,'date');
end

I can overload the operator "." by writing another function

function [value] = subsref(obj, S)

idx.type=S.type;
idx.subs={S.subs};


switch idx.type
        case '.'
                switch idx.subs{:}
                        case 'minute'
                                value=obj.minute;
                        case 'hour'
                                value=obj.hour;
                        ...
                endswitch
endswitch

end

which works fine

octave:2> min=obj.minute;
octave:3> min
min = [](0x0)
octave:4> min=obj.hour;
octave:5> min
min =  2

However, using 

builtin('subsref', obj, idx)

in

function [value] = subsref(obj, S)

idx.type=S.type;
idx.subs={S.subs};

value = builtin('subsref', obj, idx);

end

results in an error in octave-3.1.55

octave:9> obj=date(12,2,3,11,2008);
octave:10> min=obj.hour;
error: max_recursion_limit exceeded


What did I do wrong?

Thank you in advance,

Bertrand Roessli