Do I need a washer for my radiator extension?

Good evening, I'm having a bit of a battle with a radiator that I would appreciate some advice on please. I had a 950mm radiator but having failed to find one I have bought a 900mm and a 50mm fixed extension. I have fitted new radiator but it is now leaking where the extension is. Do I need a washer between the radiator and the extension. Or have I potentially got it in the wrong order - it currently is radiator straight into extension into the adapter bit into pipe.
Any help very much appreciated as my lounge is currently cold puddle.
Thanks