In this work we investigate the indium doping of CZTSe thin films. For this purpose, CZTSe was synthesized by a sequential process with different nominal In concentrations ranging from 0 to 2.6x10(20) at/cm(3). Absorbers and devices were characterized using XRF, PL, TOF-SIMS, SEM, J-V AM1.5 illuminated curves, EQE and CV. Results suggest the formation of In-Sn defects, which have a negligible impact on the carrier concentration of the absorber due to the deep character of the level introduced by this defect. This leads also to the presence of a new PL band. The main effect of the doping is reflected in changes on the morphology, where the increasing indium concentration leads to a deterioration of the absorber quality. Efficiencies in the range of 7-7.5% were obtained for In concentrations below 2.6x10(19) at/cm(3). This suggests that CZTSe is very tolerant to In doping, and high efficiency devices are obtained even with high In concentrations. A defect model based on the experimental results will be presented, explaining the apparently innocuous effect of In doping on the CZTSe electro-optical properties to a certain concentration.