diff --git a/observation_sim/sim_steps/add_pattern_noise.py b/observation_sim/sim_steps/add_pattern_noise.py
index fc091c5c09b033583509bbec70d942ba39cd5cd3..dfe90d08a76acd21aa21e620ff4deffd3317952b 100644
--- a/observation_sim/sim_steps/add_pattern_noise.py
+++ b/observation_sim/sim_steps/add_pattern_noise.py
@@ -81,5 +81,5 @@ def add_bias(self, chip, filt, tel, pointing, catalog, obs_param):
                                                nsecx=chip.nsecx,
                                                seed=self.overall_config["random_seeds"]["seed_biasNonUniform"]+chip.chipID)
     elif obs_param["bias_16channel"] == False:
-        chip.img += self.bias_level
+        chip.img += chip.bias_level
     return chip, filt, tel, pointing