MLFlow SparkTrials maxNumConcurrentTasks([]) does not exist

I'm very new at using mlflow and I'm currently having some issues on its SparkTrials. I'm running the following code in my Jupyter notebook using Anaconda:

import mlflow
from hyperopt import hp, fmin, tpe, rand, SparkTrials, STATUS_OK, STATUS_FAIL, space_eval

# replicate input_pd dataframe to workers in Spark cluster
inputs = sc.broadcast(input_pd)

# configure hyperopt settings to distribute to all executors on workers
spark_trials = SparkTrials()

# select optimization algorithm
algo = tpe.suggest

# perform hyperparameter tuning (logging iterations to mlflow)
argmin = fmin(
  fn=evaluate_model,
  space=search_space,
  algo=algo,
  max_evals=100,
  trials=spark_trials
  )

# release the broadcast dataset
inputs.unpersist()

But, I get the following error:

  Py4JError: An error occurred while calling o233.maxNumConcurrentTasks. Trace:
    py4j.Py4JException: Method maxNumConcurrentTasks([]) does not exist
        at py4j.reflection.ReflectionEngine.getMethod(ReflectionEngine.java:318)
        at py4j.reflection.ReflectionEngine.getMethod(ReflectionEngine.java:326)
        at py4j.Gateway.invoke(Gateway.java:274)
        at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:132)
        at py4j.commands.CallCommand.execute(CallCommand.java:79)
        at py4j.GatewayConnection.run(GatewayConnection.java:238)
        at java.lang.Thread.run(Unknown Source)