diff --git a/rl_coach/environments/gym_environment.py b/rl_coach/environments/gym_environment.py
index 956185c..569c537 100644
--- a/rl_coach/environments/gym_environment.py
+++ b/rl_coach/environments/gym_environment.py
@@ -63,7 +63,7 @@ class GymEnvironmentParameters(EnvironmentParameters):
         super().__init__(level=level)
         self.random_initialization_steps = 0
         self.max_over_num_frames = 1
-        self.additional_simulator_parameters = None
+        self.additional_simulator_parameters = {}
 
     @property
     def path(self):
@@ -178,7 +178,7 @@ class MaxOverFramesAndFrameskipEnvWrapper(gym.Wrapper):
 # Environment
 class GymEnvironment(Environment):
     def __init__(self, level: LevelSelection, frame_skip: int, visualization_parameters: VisualizationParameters,
-                 additional_simulator_parameters: Dict[str, Any] = None, seed: Union[None, int]=None,
+                 additional_simulator_parameters: Dict[str, Any] = {}, seed: Union[None, int]=None,
                  human_control: bool=False, custom_reward_threshold: Union[int, float]=None,
                  random_initialization_steps: int=1, max_over_num_frames: int=1, **kwargs):
         super().__init__(level, seed, frame_skip, human_control, custom_reward_threshold,
@@ -218,10 +218,12 @@ class GymEnvironment(Environment):
                 env_class = gym.envs.registration.load(self.env_id)
 
             # instantiate the environment
-            if self.additional_simulator_parameters:
+            try:
                 self.env = env_class(**self.additional_simulator_parameters)
-            else:
-                self.env = env_class()
+            except:
+                screen.error("Failed to instantiate Gym environment class %s with arguments %s" % 
+                             (env_class, self.additional_simulator_parameters), crash=False)
+                raise
         else:
             self.env = gym.make(self.env_id)