diff --git a/cartpole/log.py b/cartpole/log.py
index 1cbf678..dbbac26 100644
--- a/cartpole/log.py
+++ b/cartpole/log.py
@@ -375,7 +375,7 @@ async def register(topic_name, name, schema) -> Registration:
         async def register_class(topic_name, cls) -> Registration:
             name = cls.__name__
             assert issubclass(cls, BaseModel), f'Required pydantic model, but got {name}'
-            return await register(topic_name, name, cls.model_json_schema())
+            return await register(topic_name, name, json.dumps(cls.model_json_schema()))
 
         # preventive topic creation
         registration_log = await register(
diff --git a/cartpole/simulator.py b/cartpole/simulator.py
index 8ad9e2f..f1a6a76 100644
--- a/cartpole/simulator.py
+++ b/cartpole/simulator.py
@@ -66,7 +66,7 @@ def __init__(self, integration_step: float = 0.001):
                 cart_position=0.5, cart_velocity=2.0, cart_acceleration=5.0
             ),
             control_limit=Limits(),
-            parameters=Parameters(g=9.81, b=0, k=0.3),
+            parameters=Parameters(gravity=9.81, friction_coef=0, mass_coef=0.3),
         )
 
         self._state = State(error=Error.NEED_RESET)