title: Time Series Python : "Key Error" `start` argument could not be matched
body: <blockquote>
  <p>I have trained data set with ARIMA model ,while predicting I am
  struggling with below error.</p>
  </blockquote>
  <ul>
  <li>KeyError: 'The <code>start</code> argument could not be matched to a location
  related to the index of the data.'</li>
  </ul>
  <h2>train and valid indicate trainning and validation data set..</h2>
  <pre><code>Code:
  model = ARIMA(train, order=(2, 0, 0))
  model_fit = model.fit(disp=1)
  
  start_index = valid.index.min()
  end_index = valid.index.max()
  
  #Predictions
  predictions = model_fit.predict(start=start_index, end=end_index)
  </code></pre>