Getting Environment must inherit from the gym.Env when it already does in Stable Baselines3

Question

I have a script where I try to use stable_baselines3.__version__='2.0.0a5' methods over a custom Gym environment. The thing is that, even if my environment does inherit from the gym.Env class, I get the error AssertionError: Your environment must inherit from the gym.Env class cf https://github.com/openai/gym/blob/master/gym/core.py

This is how my environment is instantiated:

import gym
class v1(gym.Env):
     """
    GYM functions
    """
    def __init__(self):
        super().__init__()
        # My implementation of the rest of the class here

And this is how I try to use it before it fails

gym_env = v1()
print("Check env: ")
check_env(gym_env)