With MongoDB, null is returned from timestamp functions if the same function is part of WHERE clause

For example:

SELECT timestampvalue, second(timestampvalue) as sec FROM bqt1.hugeA WHERE second(timestampvalue) >= 0

This will correctly select rows where second(timestampvalue) >= 0, but the sec column in the result will contain only nulls.

If I remove the WHERE clause:

SELECT timestampvalue, second(timestampvalue) as sec FROM bqt1.hugeA;

Then the sec column will be computed correctly.