Creating Instrument Object Array for GPIB communication

Hi,

I'm trying to communicate with some lab equipment via GPIB. I kind of made it work but I'm still having some problems.

The basic code is very simple:

g=gpib('ni',0,1) %create GPIB object

out=instrfind('Type','gpib')

fopen(out)  
fprintf(out,['*IDN?'])  
fscanf(out)  
fclose(out)

The first time I run this programm works fine. But when running it again the instrument array increases by 1 and it looks like this:

 *Index:    Type:   Status:   Name:  
   1         gpib    closed    GPIB0-1
   2         gpib    closed    GPIB0-1*

and it gives the error:

??? Error using ==> icinterface.fprintf at 77 OBJ must be a 1-by-1 interface object.

And then I have to restart Matlab since I have no idea how to clear this interface object. Nothing I tried worked so far. So you can imagine it's pretty annoying.

Does anybody of you have some experience with communication via GPIB?

Thanks a lot.