Portal: Prelude Forum

I'm currently on 18 and having looked at the bunch of stuff you need to go through to get the cube,wondered if I could climb the glass wall and just take the  thing out of the dispensing hatch.
I can jump from the rear button and balance along the top of the glass until I get right in reaching grasp of the cube.No matter how close I am or if I change my position it won't be picked up. I press my 'e' button and my portal gun simply refuses to make the 'pickup' sound or pick up the cube.
Anybody else try this technique and do it successfully? Anybody else have the same problem? Is it a glitch? An intentional 'anti-short-cut' device to make you do the whole puzzle? Or am I doing it wrong?
Any thoughts appreciated.