diff --git a/src/rl4caribou/envs/caribou.py b/src/rl4caribou/envs/caribou.py
index e4068ed..d951405 100644
--- a/src/rl4caribou/envs/caribou.py
+++ b/src/rl4caribou/envs/caribou.py
@@ -191,7 +191,7 @@ def state_units(self, pop):
     def population_units(self):
         pop = (self.state + 1) * self.bound / 2
         return np.clip(
-            pop, np.repeat(0, len(pop)), np.repeat(None, len(pop))
+            pop, np.repeat(0, len(pop)), None
         )
 
 
diff --git a/src/rl4caribou/envs/caribou_ode.py b/src/rl4caribou/envs/caribou_ode.py
index b647481..5d3d6a5 100644
--- a/src/rl4caribou/envs/caribou_ode.py
+++ b/src/rl4caribou/envs/caribou_ode.py
@@ -274,7 +274,7 @@ def state_units(self, pop):
     def population_units(self):
         pop = (self.state + 1) * self.bound / 2
         return np.clip(
-            pop, np.repeat(0, len(pop)), np.repeat(None, len(pop))
+            pop, np.repeat(0, len(pop)), None
         )