diff --git a/simzoo/envs/classic_control/ex3_ekf/ex3_ekf.py b/simzoo/envs/classic_control/ex3_ekf/ex3_ekf.py
index 283fb45d..66105b36 100644
--- a/simzoo/envs/classic_control/ex3_ekf/ex3_ekf.py
+++ b/simzoo/envs/classic_control/ex3_ekf/ex3_ekf.py
@@ -231,7 +231,7 @@ def step(self, action):
         x_1, x_2 = state
 
         # Retrieve reference
-        y_1 = self.reference(x_1, x_2)
+        y_1 = self.reference(x_1)
         hat_y_1 = np.sin(hat_x_1 + self.dt * hat_x_2)
 
         # Mimic the signal drop rate