SQL: Getting null values on comparing current values to previous period

Question

I have copied this SQL query from another website and I'm not actually sure why it returns null values for the previous period and the month comparing it with. What should I adjust in the query to get the desired result?

SQL Query:

WITH monthly_metrics AS (
SELECT EXTRACT(year from day) as year,
     EXTRACT(month from day) as month,
       SUM(revenue) as revenue
  FROM daily_metrics
  GROUP BY 1,2
)
SELECT year AS current_year, 
       month AS current_month, 
       revenue AS revenue_current_month, 
       LAG(year,12) OVER ( ORDER BY year, month) AS previous_year, 
       LAG(month,12) OVER ( ORDER BY year, month) AS month_comparing_with,
       LAG(revenue,12) OVER ( ORDER BY year, month) AS revenue_12_months_ago,
       revenue - LAG(revenue,12) OVER (ORDER BY year, month) AS month_to_month_difference
FROM monthly_metrics
ORDER BY 1,2;

Query Result: dbfiddle