diff --git a/ObservationSim/sim_steps/readout_output.py b/ObservationSim/sim_steps/readout_output.py
index 1702a01914c6ba953cee77c516ab43a909858623..7e084b0d2658d6648f4db6cc905df93d3dde1d27 100644
--- a/ObservationSim/sim_steps/readout_output.py
+++ b/ObservationSim/sim_steps/readout_output.py
@@ -13,7 +13,7 @@ def add_prescan_overscan(self, chip, filt, tel, pointing, catalog, obs_param):
                                     over1=chip.overscan_x,
                                     over2=chip.overscan_y)
 
-    if obs_param["add_dark"] == True:
+    if obs_param["poisson_and_dark"]["add_dark"] == True:
         ny = int(chip.npix_y/2)
         base_dark = (ny-1)*(chip.readout_time/ny)*chip.dark_noise
         chip.img.array[(chip.prescan_y+ny):-(chip.prescan_y+ny),:] = bese_dark