diff --git a/stable_baselines3/common/callbacks.py b/stable_baselines3/common/callbacks.py
index 8f8b572..fe26199 100644
--- a/stable_baselines3/common/callbacks.py
+++ b/stable_baselines3/common/callbacks.py
@@ -470,7 +470,7 @@ class StopTrainingOnMaxEpisodes(BaseCallback):
         if self.verbose > 0 and not continue_training:
             mean_episodes_per_env = self.n_episodes / self.training_env.num_envs
             mean_ep_str = (
-                "with an average of {mean_episodes_per_env:.2f} episodes per env" if self.training_env.num_envs > 1 else ""
+                f"with an average of {mean_episodes_per_env:.2f} episodes per env" if self.training_env.num_envs > 1 else ""
             )
 
             print(