title: Azure ML Error: TimeSeriesImputer object has no attribute '_known_df'
body: <p>Running <a href="http://%20https://github.com/Azure/MachineLearningNotebooks/blob/master/how-to-use-azureml/automated-machine-learning/forecasting-orange-juice-sales/auto-ml-forecasting-orange-juice-sales.ipynb" rel="nofollow noreferrer">this orange juice sales notebook</a> I get the below error with the <code>.forecast()</code> method.</p>
  <h3>code</h3>
  <pre class="lang-py prettyprint-override"><code># The featurized data, aligned to y, will also be returned.
  # This contains the assumptions that were made in the forecast
  # and helps align the forecast to the original data
  y_predictions, X_trans = fitted_model.forecast(X_test)
  </code></pre>
  <h3>Error (<a href="https://gist.github.com/swanderz/201819978b6719bbed1826a02bb2fb47" rel="nofollow noreferrer">full stacktrace</a>):</h3>
  <pre><code>**AttributeError: 'TimeSeriesImputer' object has no attribute '_known_df'**
  </code></pre>