title: RuntimeError: The reset parameter is False but there is no n_features_in_ attribute. Is this estimator fitted?
body: <p>I am saving a sklearn.impute.SimpleImputer object using pickle. The imputer is fit with</p>
  <pre><code>imputer = SimpleImputer(missing_values = np.nan, strategy = 'mean')
  imputer = imputer.fit(train)
  </code></pre>
  <p>I then save the imputer with</p>
  <pre><code>pickle.dump(imputer,open('imputer.pkl','wb'))
  </code></pre>
  <p>and load the model in a flask app using</p>
  <pre><code>imputer = pickle.load(open('imputer.pkl','rb'))
  </code></pre>
  <p>I can successfully see the values that should be imputed if I check imputer.statistics_ . However, when I run</p>
  <pre><code>imputer.transform(test)
  </code></pre>
  <p>in my flask app using the imputer loaded with pickle, I get the following error:</p>
  <p>&quot;The reset parameter is False but there is no &quot;
  RuntimeError: The reset parameter is False but there is no n_features_in_ attribute. Is this estimator fitted?</p>
  <p>I am using sklearn version 0.23.1 to fit the imputer and transform the data. Does anyone have any insight on this? Please let me know what other information I can provide.</p>