(BBC News) The device, that also held batteries and wires, was sent to a pharmacy in the city south-west of Berlin.
Police initially said they found explosives inside, but later said there was no detonator.
Last December, 12 people died when an Islamist attacker hijacked a lorry and drove it at a Berlin Christmas market.
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The device was found near Brandenburgerstrasse in the centre of Potsdam, which has a number of other Christmas markets.