<unknown type> - numgradient and varargin

[Jose]

Hello.

I am trying to calculate gradients of a function f whose last argument 
is varargin. I am trying to use numgradient for that.

I have encountered a problem: the execution of numgradient seems to add 
an extra element to varargin (as seen by f) with unknown type.

Have a look:

----->
octave:16> clear all
octave:17> pkg load optim
octave:18> function a = f(x,varargin);
> nargin
> a = x;
> endfunction
octave:19> args{1}=2;
octave:20> numel(args)
ans =  1
octave:21> f(1,args{:});
ans =  2
octave:22> numgradient('f',{1,args{:}});
ans =  3
ans =  3
ans =  3
octave:23> function a = f(x,varargin);
> varargin
> a = x;
> endfunction
octave:24> f(1,args{:});
varargin =
{
  [1,1] =  2
}
octave:25> numgradient('f',{1,args{:}});
varargin =
{
  [1,1] =  2
  [1,2] =

error: octave_base_value::print (): wrong type argument '<unknown type>'

}
error: called from:
error:   f at line 2, column 1
error: octave_base_value::matrix_value(): wrong type argument '<unknown 
type>'
error: octave_base_value::matrix_value(): wrong type argument '<unknown 
type>'
error: octave_base_value::matrix_value(): wrong type argument '<unknown 
type>'
octave:25>
<-----

I am using octave 3.8.1 and optim-1.3.0.

Am I missing something or should I file a bug?

Regards
J.