diff --git a/flatland/envs/observations.py b/flatland/envs/observations.py
index 22ebf264183528762fb9b4c0c6bea07bfc726423..2bf1b55c91ea69302be7bdf2385c09198345e156 100644
--- a/flatland/envs/observations.py
+++ b/flatland/envs/observations.py
@@ -420,7 +420,8 @@ class TreeObsForRailEnv(ObservationBuilder):
                 for i in range(self.max_depth - depth):
                     num_cells_to_fill_in += pow4
                     pow4 *= 4
-                observation = observation + [-np.inf, -np.inf, -np.inf, -np.inf, -np.inf, -np.inf] * num_cells_to_fill_in
+                observation = \
+                    observation + [-np.inf, -np.inf, -np.inf, -np.inf, -np.inf, -np.inf] * num_cells_to_fill_in
 
         return observation, visited