diff --git a/flatland/envs/observations.py b/flatland/envs/observations.py
index 5c4ed86bdb4517d9501a887a6be6f256a7297e7e..bd095dba527dae03e7ccc35b09c19039119ed6c9 100644
--- a/flatland/envs/observations.py
+++ b/flatland/envs/observations.py
@@ -479,7 +479,7 @@ class TreeObsForRailEnv(ObservationBuilder):
         """
 
         if len(tree) < num_features_per_node:
-            return [], []
+            return [], [], []
 
         depth = 0
         tmp = len(tree) / num_features_per_node - 1