diff --git a/observation_sim/sim_steps/add_pattern_noise.py b/observation_sim/sim_steps/add_pattern_noise.py
index dfe90d08a76acd21aa21e620ff4deffd3317952b..f92cf36be89a62dfbb642a4643a8a4ea49a1b6e5 100644
--- a/observation_sim/sim_steps/add_pattern_noise.py
+++ b/observation_sim/sim_steps/add_pattern_noise.py
@@ -27,7 +27,7 @@ def add_poisson_and_dark(self, chip, filt, tel, pointing, catalog, obs_param):
                                              InputDark=None)
     else:
         chip.img, _ = chip_utils.add_poisson(img=chip.img,
-                                             chip=self,
+                                             chip=chip,
                                              exptime=exptime,
                                              poisson_noise=chip.poisson_noise,
                                              dark_noise=0.)