agents: Error loading DqnAgent saved model.

I am creating a tf-agent DqnAgent in the following code:

    tf_agent = dqn_agent.DqnAgent(
        train_env.time_step_spec(),
        train_env.action_spec(),
        q_network=q_net,
        optimizer=optimizer,
        td_errors_loss_fn=dqn_agent.element_wise_squared_loss,
        train_step_counter=train_step_counter
)

During the training loop I am saving this model with

    tf.saved_model.save(tf_agent, saved_models_path)

Once trained, I want to load saved model with

    if tf.saved_model.contains_saved_model(saved_models_path):
        tf_agent = tf.saved_model.load(saved_models_path)

This code will load the saved model only if the folder in saved_path contains one, the functions contains_saved_model(saved_models_path) returns True, so the model is loaded, but there is an excetion and the program crashes:

    Traceback (most recent call last):
        File "/home/claudino/Projetos/dino-tf-agents/dino_ia/model/agent.py", line 50, in <module>
            tf_agent = tf.saved_model.load(saved_models_path)
        File "/home/claudino/Projetos/dino-tf-agents/venv/lib/python3.6/site-packages/tensorflow/python/saved_model/load.py", line 408, in load
            return load_internal(export_dir, tags)
        File "/home/claudino/Projetos/dino-tf-agents/venv/lib/python3.6/site-packages/tensorflow/python/saved_model/load.py", line 432, in load_internal
            export_dir)
        File "/home/claudino/Projetos/dino-tf-agents/venv/lib/python3.6/site-packages/tensorflow/python/saved_model/load.py", line 58, in __init__
            self._load_all()
        File "/home/claudino/Projetos/dino-tf-agents/venv/lib/python3.6/site-packages/tensorflow/python/saved_model/load.py", line 168, in _load_all
            slot_variable = optimizer_object.add_slot(
        AttributeError: '_UserObject' object has no attribute 'add_slot'

        Process finished with exit code 1