diff --git a/trl/trainer/ppo_trainer.py b/trl/trainer/ppo_trainer.py
index 70912fb336c..5e941a181e8 100644
--- a/trl/trainer/ppo_trainer.py
+++ b/trl/trainer/ppo_trainer.py
@@ -146,9 +146,7 @@ class PPOTrainer(BaseTrainer):
     def __init__(
         self,
         args: PPOConfig,
-        processing_class: Optional[
-            Union[PreTrainedTokenizerBase, BaseImageProcessor, FeatureExtractionMixin, ProcessorMixin]
-        ],
+        processing_class: Union[PreTrainedTokenizerBase, BaseImageProcessor, FeatureExtractionMixin, ProcessorMixin],
         model: nn.Module,
         ref_model: Optional[nn.Module],
         reward_model: nn.Module,