title: Why are my BQML evaluation metrics all NaN?
body: <p>I create a time series model:</p>
  <pre><code>CREATE OR REPLACE MODEL mymodel
  
  OPTIONS(
    MODEL_TYPE='ARIMA_PLUS',
    TIME_SERIES_TIMESTAMP_COL='date', 
    TIME_SERIES_DATA_COL='cost',
    TIME_SERIES_ID_COL='grp',
    HOLIDAY_REGION='GB'
  ) AS
  
  SELECT grp, date, cost FROM mydata
  </code></pre>
  <p>I then evaluate it:</p>
  <pre><code>SELECT *
  FROM
    ML.EVALUATE(MODEL mymodel,
      (SELECT grp, date, cost FROM mydata),
      STRUCT(FALSE AS perform_aggregation, 0.9 AS confidence_level,
      12 AS horizon))
  </code></pre>
  <p>But all my metrics and forecasted value have value <code>NaN</code>. Could it be I am using the wrong <code>input</code> data (ie <code>(SELECT grp, date, cost FROM mydata)</code> in the above)? The <a href="https://cloud.google.com/bigquery-ml/docs/reference/standard-sql/bigqueryml-syntax-evaluate" rel="nofollow noreferrer">docs</a> say it is the data <code>that contains the evaluation data</code> which is unclear to me.</p>