diff --git a/com.unity.ml-agents/CHANGELOG.md b/com.unity.ml-agents/CHANGELOG.md
index 26102b8958..0237b6c34e 100755
--- a/com.unity.ml-agents/CHANGELOG.md
+++ b/com.unity.ml-agents/CHANGELOG.md
@@ -19,6 +19,7 @@ and this project adheres to
 #### ml-agents / ml-agents-envs
 - Fixed a bug when using LSTM and SAC where the buffer might contain non-integer numbers of sequences. (#6301)
 - Fixed mixed CPU/GPU computation by ensuring tensors share a consistent device in the trainer; Updated training devices. (#6303)
+- Fix typing on optional strings (#6311)
 
 #### Examples
 - Replace hardcoded value 200f by agentRotationSpeed in MoveAgent() (#6306)
diff --git a/ml-agents-envs/mlagents_envs/timers.py b/ml-agents-envs/mlagents_envs/timers.py
index 32b8602c9c..f230730338 100644
--- a/ml-agents-envs/mlagents_envs/timers.py
+++ b/ml-agents-envs/mlagents_envs/timers.py
@@ -71,7 +71,10 @@ def add_time(self, elapsed: float) -> None:
         self.count += 1
 
     def merge(
-        self, other: "TimerNode", root_name: str = None, is_parallel: bool = True
+        self,
+        other: "TimerNode",
+        root_name: Optional[str] = None,
+        is_parallel: bool = True,
     ) -> None:
         """
         Add the other node to this node, then do the same recursively on its children.
diff --git a/ml-agents/mlagents/trainers/directory_utils.py b/ml-agents/mlagents/trainers/directory_utils.py
index 80379d81e9..5e979fab93 100644
--- a/ml-agents/mlagents/trainers/directory_utils.py
+++ b/ml-agents/mlagents/trainers/directory_utils.py
@@ -1,11 +1,12 @@
 import os
+from typing import Optional
 from mlagents.trainers.exception import UnityTrainerException
 from mlagents.trainers.settings import TrainerSettings
 from mlagents.trainers.model_saver.torch_model_saver import DEFAULT_CHECKPOINT_NAME
 
 
 def validate_existing_directories(
-    output_path: str, resume: bool, force: bool, init_path: str = None
+    output_path: str, resume: bool, force: bool, init_path: Optional[str] = None
 ) -> None:
     """
     Validates that if the run_id model exists, we do not overwrite it unless --force is specified.
diff --git a/ml-agents/mlagents/trainers/trainer/trainer_factory.py b/ml-agents/mlagents/trainers/trainer/trainer_factory.py
index ffb7741513..c5b0cfd49d 100644
--- a/ml-agents/mlagents/trainers/trainer/trainer_factory.py
+++ b/ml-agents/mlagents/trainers/trainer/trainer_factory.py
@@ -1,5 +1,5 @@
 import os
-from typing import Dict
+from typing import Dict, Optional
 
 from mlagents_envs.logging_util import get_logger
 from mlagents.trainers.environment_parameter_manager import EnvironmentParameterManager
@@ -23,7 +23,7 @@ def __init__(
         load_model: bool,
         seed: int,
         param_manager: EnvironmentParameterManager,
-        init_path: str = None,
+        init_path: Optional[str] = None,
         multi_gpu: bool = False,
     ):
         """