title: Parameter values for parameter (n_estimators) need to be a sequence
body: <p>I am getting the error, not sure how to fix it. Can you please help. The entire code can be found at <a href="https://github.com/kthouz/NYC_Green_Taxi/blob/master/NYC%20Green%20Taxi.ipynb" rel="noreferrer">https://github.com/kthouz/NYC_Green_Taxi/blob/master/NYC%20Green%20Taxi.ipynb</a></p>
  
  <h1>optimize n_estimator through grid search</h1>
  
  <pre><code>def optimize_num_trees(alg,param_test,scoring_method,train,predictors,target):
      """
      This functions is used to tune paremeters of a predictive algorithm
      alg: sklearn model,
      param_test: dict, parameters to be tuned
      scoring_method: str, method to be used by the cross-validation to valuate the model
      train: pandas.DataFrame, training data
      predictors: list, labels to be used in the model training process. They should be in the column names of dtrain
      target: str, target variable
      """
      gsearch = GridSearchCV(estimator=alg, param_grid = param_test, scoring=scoring_method,n_jobs=2,iid=False,cv=5)
      gsearch.fit(train[predictors],train[target])
      return gsearch
  
  # get results of the search grid
  gs_cls = optimize_num_trees(model_cls,param_test,'roc_auc',train,predictors,target)
  
  ---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-42-c7419a90cdb1&gt; in &lt;module&gt;()
        1 
        2 # get results of the search grid
  ----&gt; 3 gs_cls = optimize_num_trees(model_cls,param_test,'roc_auc',train,predictors,target)
        4 
  
  &lt;ipython-input-40-2b76f2ffb87f&gt; in optimize_num_trees(alg, param_test, scoring_method, train, predictors, target)
       57     target: str, target variable
       58     """
  ---&gt; 59     gsearch = GridSearchCV(estimator=alg, param_grid = param_test, scoring=scoring_method,n_jobs=2,iid=False,cv=5)
       60     gsearch.fit(train[predictors],train[target])
       61     return gsearch
  
  /Users/anaconda/lib/python3.5/site-packages/sklearn/grid_search.py in __init__(self, estimator, param_grid, scoring, fit_params, n_jobs, iid, refit, cv, verbose, pre_dispatch, error_score)
      810             refit, cv, verbose, pre_dispatch, error_score)
      811         self.param_grid = param_grid
  --&gt; 812         _check_param_grid(param_grid)
      813 
      814     def fit(self, X, y=None):
  
  /Users/anaconda/lib/python3.5/site-packages/sklearn/grid_search.py in _check_param_grid(param_grid)
      346             if True not in check:
      347                 raise ValueError("Parameter values for parameter ({0}) need "
  --&gt; 348                                  "to be a sequence.".format(name))
      349 
      350             if len(v) == 0:
  
  ValueError: Parameter values for parameter (n_estimators) need to be a sequence.
  </code></pre>