Why IS NULL in WHERE clause is not working in BigQuery?

I have just started practicing Big Query but facing a problem for filtering out non-null records.

I am using usertype IS NULL in the WHERE clause, it throws no error just says that "There is no data to display." but there are multiple null records in a dataset.

ColName    Type     Mode
---------------------------    
usertype   STRING   NULLABLE

Query:

SELECT *
FROM `bigquery-public-data.new_york_citibike.citibike_trips`
WHERE usertype IS NULL;

Result:

There is no data to display.

Can anyone please help?