First of all, test your potentiostat with a linear sweep voltammetry (-0.1V to +0.1 V, CE and RE connected together) on a resistor (1-10 kohm). In your record, the potential must be below +/_1 mV for i=0. If this is the case, read this paper:

http://jes.ecsdl.org/content/164/13/C819.full.pdf+html

(also attached here).

The general solution and a lot of useful hints are there.