diff --git a/vivarium/environments/braitenberg/selective_sensing/init.py b/vivarium/environments/braitenberg/selective_sensing/init.py
index 192af4a..34303dd 100644
--- a/vivarium/environments/braitenberg/selective_sensing/init.py
+++ b/vivarium/environments/braitenberg/selective_sensing/init.py
@@ -98,6 +98,13 @@ def get_agents_params_and_sensed_arr(agents_stacked_behaviors_list):
     return params, sensed, behaviors
 
 def get_positions(positions, n, box_size):
+    """Check if the positions are valid and return them if they are
+
+    :param positions: positions of the entities
+    :param n: number of entities
+    :param box_size: size of the box
+    :return: positions
+    """
     if positions is None:
         return [None] * n
     assert len(positions) == n, f"The number of positions: {len(positions)} must match the number of entities: {n}"
@@ -107,6 +114,12 @@ def get_positions(positions, n, box_size):
     return positions
 
 def check_position_redundancies(agents_pos, objects_pos):
+    """Check if there are redundant positions in the agents and objects positions
+
+    :param agents_pos: agents positions
+    :param objects_pos: objects positions
+    :return: redundant_positions
+    """
     positions = agents_pos + objects_pos
     position_indices = defaultdict(list)
 
@@ -119,13 +132,24 @@ def check_position_redundancies(agents_pos, objects_pos):
     return redundant_positions if (len(redundant_positions) > 0) else False
 
 def get_exists(exists, n):
+    """Check if the exists array is valid and return it if it is
+
+    :param exists: exists array
+    :param n: number of entities
+    :return: exists
+    """
     if exists is None:
         return [1] * n
-    assert isinstance(exists, int) and (exists < n), f"Exists must be an int inferior than {n}, {exists} is not"
+    assert isinstance(exists, int) and (exists <= n), f"Exists must be an int inferior or equal than {n}, {exists} is not"
     exists = [1] * exists + [None] * (n - exists)
     return exists
 
 def set_to_none_if_all_none(lst):
+    """Set the list to None if all elements are None
+
+    :param lst: list to check
+    :return: lst
+    """
     if not any(element is not None for element in lst):
         return None
     return lst
@@ -286,7 +310,6 @@ def init_objects(
         color=objects_color
     )
 
-
 def init_complete_state(
     entities,
     agents,
@@ -317,8 +340,13 @@ def init_complete_state(
         ent_sub_types=total_ent_sub_types
     )   
 
-
 def process_entity(data, box_size):
+    """Process the entity data to extract the color, positions, exists and diameter
+
+    :param data: entity data
+    :param box_size: box size
+    :return: entity_data, positions, exists, diameter_lst
+    """
     n = data['num']
     color_str = data['color']
     color = _string_to_rgb_array(color_str)
@@ -328,7 +356,7 @@ def process_entity(data, box_size):
     diameter_lst = [diameter] * n
     return {'n': n, 'color': color}, positions, exists, diameter_lst
 
-
+# TODO : should refactor all the yaml configs to only define a dict of agents and a dict of objects, instead of specifying it for each subtype of entities
 def init_state(
     entities_data=CONFIG.entities_data,
     box_size=CONFIG.box_size,
@@ -360,6 +388,11 @@ def init_state(
     ent_sub_types_enum = Enum('ent_sub_types_enum', {ent_sub_types[i]: i for i in range(len(ent_sub_types))}) 
     ent_data = entities_data['Entities']
 
+    # check if at least one agent and one object are defined in the entities data
+    has_agent, has_object = check_agent_and_object(ent_data)
+    assert has_agent, "At least one agent must be defined in the entities data"
+    assert has_object, "At least one object must be defined in the entities data"
+
     # create max agents and max objects
     max_agents = 0
     max_objects = 0 
@@ -529,3 +562,18 @@ def init_state(
 
     return state
 
+def check_agent_and_object(ent_data):
+    has_agent = False
+    has_object = False
+
+    for entity in ent_data.values():
+        if entity['type'] == 'AGENT':
+            has_agent = True
+        elif entity['type'] == 'OBJECT':
+            has_object = True
+
+        # If both are found, no need to continue checking
+        if has_agent and has_object:
+            break
+
+    return has_agent, has_object