How to preserve NULL value with q2a queries?

Question

As far as I can see, this query:

qa_db_query_sub('UPDATE `qa_users` SET '.$field.' = $ WHERE userid = # ',
$value, $userid);

insert a string for $field when the passed $value is NULL.

How can I force the NULLing of the field?

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Update: 

Seems that I had a trim($value) somewhere that made the NULL an empty string. 

So NULL is set in the DB using the query above!