diff --git a/sbx/ppo/ppo.py b/sbx/ppo/ppo.py
index 46eb5d7..4762ae7 100644
--- a/sbx/ppo/ppo.py
+++ b/sbx/ppo/ppo.py
@@ -259,7 +259,7 @@ def train(self) -> None:
         # Update optimizer learning rate
         if self.target_kl is None:
             self._update_learning_rate(
-                [self.policy.actor_state.opt_state[1], self.policy.vf_state.opt_state[1]],
+                [self.policy.actor_state.opt_state[-1], self.policy.vf_state.opt_state[-1]],
                 learning_rate=self.lr_schedule(self._current_progress_remaining),
             )
         # Compute current clip range
@@ -311,7 +311,7 @@ def train(self) -> None:
                     self.adaptive_lr.update(approx_kl_div)
 
                     self._update_learning_rate(
-                        [self.policy.actor_state.opt_state[1], self.policy.vf_state.opt_state[1]],
+                        [self.policy.actor_state.opt_state[-1], self.policy.vf_state.opt_state[-1]],
                         learning_rate=self.adaptive_lr.current_adaptive_lr,
                     )
         self._n_updates += self.n_epochs
diff --git a/tests/test_ppo.py b/tests/test_ppo.py
new file mode 100644
index 0000000..2532210
--- /dev/null
+++ b/tests/test_ppo.py
@@ -0,0 +1,110 @@
+from typing import Optional, Any
+
+import flax.linen as nn
+import numpy as np
+import pytest
+from stable_baselines3.common.env_util import make_vec_env
+import optax
+import jax
+from gymnasium import spaces
+from flax.training.train_state import TrainState
+import jax.numpy as jnp
+
+from sbx import PPO
+
+from sbx.ppo.policies import PPOPolicy, Actor, Critic
+
+
+
+class CustomPPO(PPOPolicy):
+    def __init__(self, observation_space, action_space, lr_schedule, net_arch = None, ortho_init = False, log_std_init = 0, activation_fn = nn.tanh, use_sde = False, use_expln = False, clip_mean = 2, features_extractor_class=None, features_extractor_kwargs = None, normalize_images = True, optimizer_class = optax.adam, optimizer_kwargs = None, share_features_extractor = False, actor_class = Actor, critic_class = Critic):
+        super().__init__(observation_space, action_space, lr_schedule, net_arch, ortho_init, log_std_init, activation_fn, use_sde, use_expln, clip_mean, features_extractor_class, features_extractor_kwargs, normalize_images, optimizer_class, optimizer_kwargs, share_features_extractor, actor_class, critic_class)
+
+    
+    def build(self, key: jax.Array, lr_schedule, max_grad_norm) -> jax.Array:
+
+        # Coustom PPO Policy build  
+
+        key, actor_key, vf_key = jax.random.split(key, 3)
+        key, self.key = jax.random.split(key, 2)
+        self.reset_noise()
+
+        obs = jnp.array([self.observation_space.sample()])
+
+        if isinstance(self.action_space, spaces.Box):
+            actor_kwargs: dict[str, Any] = {
+                "action_dim": int(np.prod(self.action_space.shape)),
+            }
+        elif isinstance(self.action_space, spaces.Discrete):
+            actor_kwargs = {
+                "action_dim": int(self.action_space.n),
+                "num_discrete_choices": int(self.action_space.n),
+            }
+        elif isinstance(self.action_space, spaces.MultiDiscrete):
+            assert self.action_space.nvec.ndim == 1, (
+                "Only one-dimensional MultiDiscrete action spaces are supported, "
+                f"but found MultiDiscrete({(self.action_space.nvec).tolist()})."
+            )
+            actor_kwargs = {
+                "action_dim": int(np.sum(self.action_space.nvec)),
+                "num_discrete_choices": self.action_space.nvec,  # type: ignore[dict-item]
+            }
+        elif isinstance(self.action_space, spaces.MultiBinary):
+            assert isinstance(self.action_space.n, int), (
+                f"Multi-dimensional MultiBinary({self.action_space.n}) action space is not supported. "
+                "You can flatten it instead."
+            )
+            # Handle binary action spaces as discrete action spaces with two choices.
+            actor_kwargs = {
+                "action_dim": 2 * self.action_space.n,
+                "num_discrete_choices": 2 * np.ones(self.action_space.n, dtype=int),
+            }
+        else:
+            raise NotImplementedError(f"{self.action_space}")
+
+        self.actor = self.actor_class(
+            net_arch=self.net_arch_pi,
+            log_std_init=self.log_std_init,
+            activation_fn=self.activation_fn,
+            ortho_init=self.ortho_init,
+            **actor_kwargs,  # type: ignore[arg-type]
+        )
+
+        self.actor.reset_noise = self.reset_noise
+        optimizer_class = optax.inject_hyperparams(self.optimizer_class)(learning_rate=lr_schedule(1), **self.optimizer_kwargs)
+
+        self.actor_state = TrainState.create(
+            apply_fn=self.actor.apply,
+            params=self.actor.init(actor_key, obs),
+            tx=optax.chain(
+                # optax.clip_by_global_norm(max_grad_norm), # ->  Test an Optax chain with only one element.
+                optimizer_class,
+            ),
+        )
+
+        self.vf = self.critic_class(net_arch=self.net_arch_vf, activation_fn=self.activation_fn)
+
+        self.vf_state = TrainState.create(
+            apply_fn=self.vf.apply,
+            params=self.vf.init(vf_key, obs),
+            tx=optax.chain(
+                # optax.clip_by_global_norm(max_grad_norm), # -> Test an Optax chain with only one element.
+                optimizer_class,
+            ),
+        )
+
+        self.actor.apply = jax.jit(self.actor.apply)  # type: ignore[method-assign]
+        self.vf.apply = jax.jit(self.vf.apply)  # type: ignore[method-assign]
+
+        return key 
+
+def test_ppo() -> None:
+    env = make_vec_env('Pendulum-v1')
+
+    # PPO assumes that the train state (self.vf_state) is created from an Optax chain with two elements.
+    model = PPO(
+        CustomPPO,
+        env
+    )
+    
+    model.learn(64, progress_bar=True)